My path may not have been optimal...I was a thief. But here goes:
1-12: Soloing in West and East Sarutabaruta, pretty much experimenting fighting anything to learn your limits.
12-15: From here on, you need an optimal party of good and competent people to make the big XP. With that in mind, Maze Makers in Shakhrami are good at this time. You can also party up against Dhalmels for an easier time and some good money items in the mid-portions of the canyon. If you're still soloing sometimes, as I was, you will have trouble at some point where Rarabs/Saplings/Pygmaoi are too easy, but stuff further into the canyon is too dangerous. Akbabas make for a nice bridge. They hang out near the giant bone sites, such as the SW corner. (Daytime only-- ghosts there make it too hazardous when night falls!)
15-20: The options explode here. I stuck around in the Canyon quite late, reducing party size to keep exp high until I was nearly soloing again. Did a few parties in Meriphataud, going for Jubjubs or Hill Lizards. But I now know that in a good full party you can start doing Valkurm or Buburimu at this time too. Some start even earlier, but 15 seems safest and allows you to get a head start on your subjob item collections.
A brief time out to talk about the support job quests: As you know, there are two quests. You can take either, but I do not believe you can take both. Therefore, to avoid getting locked into a bad choice, do not talk to either quest offerer until you have all the right items.
Mhaura Quest: This one might be faster, but is
much harder. You need a wild rabbit tail, a cup of dhalmel saliva, and a bloody robe from creatures of Buburimu. You get the wild rabbit tail from the Mighty Rarab. You get the dhalmel saliva from the Bull Dhalmel. And, the last item is really hard. You need a bloody robe from a Bogy, which appear at night near the lighthouse rocks of Buburimu. Bogies will even make people many levels above you tremble in fear and are not to be trifled with. Being a ghost type, they have a number of nasty attacks, are resistant to melee, and will cast "Curse", lowering your max HP and MP. They can be taken by a party as early as 25, but I can't guarantee someone won't die. Much better you just make friends with a Lv. 39+, have him party with you, stand back while he kills Bogies until one drops the robe, and let you have it. Talk to Vera in Mhaura and give them to her as she asks for them.
Selbina Quest: This one is more popular because it's doable entirely by people at and around Lv. 18. You need a crab apron, a damselfly worm, and a magicked skull from the creatures of Valkurm. You can get crab aprons from Snippers down by the beach. You can get damselfly worms from Damselflies. The last item is hardest, and is bound to be more so on this server what with the crowds. You need a magicked skull from a Ghoul. Not only is this a rare drop, but ghouls only appear at night, and only near trees (oases). They're sure to be heavily camped. To make the ghoul appear near the oases, it helps to kill other creatures nearby because there's a small chance that a ghoul will spawn to replace it later. And those goblins are good experience. An alternative is Gusgen Mines, but those are heavily camped too, the ghouls tend to respawn too close and too quickly, they are harder, and they drop skulls even less often than in Valkurm. Talk to Isacio in Selbina and give them to him as he asks for them.
Okay, back to levelling soon. But first, you'll want your chocobo license. And for that, you'll need four clumps of gausebit wildgrass. You can choose how to get them: Wadi Hares in the middle-upper rooms of Dangruf Wadi, Crane Flies in the NE of Meriphataud, or buying the grass. The hares are easy, but the goblins that roam there will make life hard for you. Go with a friend or two...it'll take a lot of hare-killing to pull this off. To make things easier, take a boomerang so you can pull hares without alerting goblins nearby. An alternative that's harder, but has higher drop frequency is to kill Crane Flies in Meriphataud Mountains. You'll probably need a party, but they drop grass far more often than the hares, it seems, so it should balance out. And of course, there's the expensive option which is to just buy the grass at the AH.
Now comes the scary part. You're going to need to run to Jeuno, though areas with mobs far too hard for you. And if you die and delevel below 20 on your way, you can't do the chocobo license quest anymore. So...first do yourself a favor and get yourself a full set of maps for the overland areas. The last thing you need is to get lost on the way there. The next thing you want to do is go to your moghouse and switch to a job you have not used. These enemies are far too hard to fight, and it really hardly matters whether you're Lv. 1 or Lv. 20...you might survive one hit at Lv. 20...but then what? You die and lose experience. As a Lv. 1, you have nothing to lose. I suggest THF if you haven't levelled it, because it lets you see enemies as red dots in the compass display, and you can focus on keeping those dots outside the compass circle. There are three paths, and everyone has a favorite. Here's my opinion:
Meriphataud to Sauromugue to Jeuno: This is a pretty good one. Most of Meriphataud can be run through deep trenches, out of the sight of enemies. Sauromugue is harder though, it's wide open, but the wide open spaces allow enemies to roam widely and quickly. Take it very slowly and stay near the path on the map. The only things that do not aggro here are bats, lizards, and beetles.
Jugner to Battalia to Jeuno: This is a hard one in my opinion, but a lot of people like it. Jugner is the hard part here. Tigers move with surprising speed, and orcs and goblins roam the area freely. Moss hanging from tree branches makes it very hard to see ahead. Saplings, birds, pugils, crabs, sheep, and leeches do not aggro. I suggest staying to the wide winding gulley where the path is on the map. Once in Battalia, things are more dangerous, but you can see further. Saplings, crabs, flies, and birds are safe. Position your camera so nothing surprises you over these tall hills and don't trust the thief radar so much because you can't see things above or below you so clearly.
Pashhow to Rolanberry to Jeuno: This is the one I did. It's hard, but I know a lot of people who did this one just fine. Passhow is easy if you go slowly and wait out the aggro mobs until they move off to one side. Rabbits, bats, bees, pugils, crabs, flies, and funguars are safe. Stick to the widest S-shaped path through the middle...the shortcuts are tempting and give you no room for avoiding enemies. Rolanberry is harder, so go even slower. Bats, bees, and Berry Grubs are safe. Keep in sight of the path as before, and stick close, deviating from the path only as needed to avoid enemies. You can run through vineyards if you need to...only the stakes will block you. Just move slowly and don't be afraid to wait for enemies to move out of your way.
With luck, you're now in Jeuno. Now you can talk to Brutus at the Upper Jeuno chocobo stables and start the quest. Attempt to feed Gausebit grass to the abused chocobo once each game day. It will refuse twice and eat four times, at which point you'll get the license.
20-24: Finally, time to level some more! As luck would have it, Qufim Island, at the base of Jeuno is an excellent place. Just head right down the stairs from Port Jeuno and you'll find yourself in a melting ice cave. It may require a White Mage to cast Sneak on you getting there because the tunnel is narrow and has several places where Dancing Weapons hang out. Once you get through the tunnel, you'll be in a high-walled canyon, where you can gather for a party. You may want to sit back in the tunnel a ways if the area isn't too heavily populated, since there are a couple of worms that pop up from time to time and will attack if you're too close. Once you have a party though, those worms are the reason to be in Qufim. They give exellent experience. The island is overrun with giants too, but if you can get around them, there's a safe spot in the middle of the island around a lake where you can kill worms without giants interfering. Just make sure you watch the clock and retreat to the canyon at the entrance before dark. At night, Wights and Banshees come out, and they're very deadly at this level so retreat and huddle until dawn.
24-28: Head deeper into Qufim, to the area just outside the base of Delkfutt's Tower. You can zone into the tower in an emergency. Start with Clippers, then work up to Greater Pugils. Watch out for Wights, as they're just as active and aggressive here, plus you now frequently get elementals wandering around harrassing mages, and a few giants for good measure. Be aware of your surroundings. For a change of scenery, you can enter the tower, take the first right, and try pulling the goblins one at a time from the next few rooms. Around 29, you can extend your fun with Acrophies in a good party, but at this point, you should probably start moving on.
28-32: For a short time, a good and profitable place to fight is in Battalia Downs. The many mounds in the area have chambers underneath them, and about half of those chambers connect to an area called Eldieme Necropolis. You're not ready for Eldieme yet, but just as Delkfutt's Tower served as a good place to escape if things got nasty in Qufim, Eldieme will give you a zone for escape should a battle go sour. So hang out in Battalia in one of those underground chambers and send someone out to pull sabertooth tigers, goblins, and orcs down one by one. Tigers aren't such great exp, but their hides and teeth fetch good money.
32-34: Once you get a taste for orcs, you can get a lot more of them in Davoi. Following the right wall from the entrance is a great rope bridge over which you can pull orcs one at a time. But Davoi can get crowded, and if too many novice parties are messing up and pulling too hard an orc, you'll find yourself running from trains more often than levelling. An alternate location if you're sick of Davoi is Beadeaux. I also hear Knight Crabs in Jugner are good if you can pull off chains.
34-40: I spent much of my time in this period in Crawler's Nest fighting Worker Crawlers, and perhaps Nest Beetles later on, though they're harder to pull, and common wisdom is that you want a BLM with you to take them on at all (though I was in more than a couple of parties that ignored that wisdom and survived to tell the tale). It's good provided you have a good puller and nobody trains them all to the entrance. An alternative is Eldieme Necropolis, fighting Gazers and Lost Souls. The nice thing about the necropolis beasts is that, while aggro, they supposedly do not link, so you can pull them one by one from a safe non-aggro distance.
40-45: Deeper in Crawler's Nest, you'll find Soldier Crawlers. They're aggro, but fairly easy to pull. Just watch out for Exorays and make sure you have Escape, Teleport, or Warp II in the party in case you do get in trouble. This deep, you can train the entire zone and annoy a lot of people if you try to run for it. (Raise your hand if you remember the time someone trained an Exoray up in CN.
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Anyhow...that's about as far as I got in beta. I gathered that Quicksand Caves is a good place to try around 45, but I never got the chance to try. And of course, don't be afraid to deviate from this. Things are much different in retail. We did not have access to the expansion areas until we reached Level 40. Granted, most of said areas are not really doable at less than Level 45, but some people seemed very excited at seeing Land Worms in Korroloka Tunnel, raising the possibility of it being an alternative to Qufim had we had access to it at Lv. 20, for example. And the population of the server is about three times what it was in beta, so some popular areas may be overcrowded, leading to more fruitful results trying the less well-known areas.
j/k