exp have been buffed immensely compared to the time as ARR came out. Here's what I do with my alts most of the time, and that's just a suggestion nothing that you need to do 100%.
I start in the city as a new character, pick up every quest i find (lvl 1-3 mostly), get all the aether points (city teleports) and follow the main scenario quest. The reason I do that is, you're running that way anyways, might as well get some exp out of it.
After I've done all that, you have at least 2+ quests that tell you to go beat up some mobs. So now pick a direction (doesn't matter which one), go do that quest, as a hint most of the mobs are in different level ranges. Example, there could be a lady bug that's level 1 and a bit further down the road there could be one that's level 2 or 3. I always get these, because how the system works at the moment, if you kill 2 of them, you will get under your exp bar a timer, that indicates that for a specific amount of time you get more exp. So after a couple of minutes going that route, you will be level 5.
Now you have unlocked the hunting log, that gives another bonus for defeating specific kind of mobs, most of them are in the same level range. Example, your hunting log wants you to kill 3 Aurelli's, since you need to kill them anyways for your other quest, head in that direction, kill 3 or 4 and get head back to the city. Make sure that you don't return to the city every single time you've finished a quest. It's wasting time running forth and back, just go back when you finished all of them. Make sure that you do not forget your main scenario quest on the way as well.
Go ahead and do all of these quests in different zones and your hunting logs, fates you run by that are not bosses etc. One thing I would suggest tho, pick up quests only when passing by or you see that you're going in that direction. Don't get out of your way finish all these quests. Also don't forget your class/job quest you're getting every 5 levels. That way you can get some cheap armor/weapons and won't have to bother with npc's or the market board.
If we following that method, you should be level 12-14 by now. Stop doing your quests, only pick up or finish that main scenario quest. Ignore everything else! There's a quest at level 15 (main scenario) that takes you to the different cities. Now here is were it sounds complicated but it isn't. So let's say you started in Ulda, the quest takes you to Limsa next. Talk to the npc at the air landing to update that quest step. Ignore that quest for now. Go down the elevator or take the stairs lol, and pick up a new class if you're interested in one that's offered (Ninja, Arcanist, Marauder).
If you picked up a class that was from interest, switch to that class and repeat all the city things with that class and not your main class. (city teleports, city quests, class/job quests etc). You can ignore the quests outside these cities or do them, whatever you want, or how much time you like to invest. If you didn't, than get at least all the inner city teleports with your main character, and don't forget to set favorite or free teleport, or you are going to pay an arm and a leg later on.
After you have done that, switch back/stay to/on your main class and follow up with your main quest. It takes you to Gridania, same thing here, just rinse and repeat. Finish the quest and it will tell you to run some errands, and than points you into one direction... your first dungeon (Satasha). Now before you run there, make sure you picked up your new gear from the novice hall (30% exp ring) and pick up a leve if you haven't done that yet, it will unlock the guild hest via duty finder and that's a good chunk of exp as well, especially for your classes later on.
Done all that? Great now run every main scenario quest with your main class from now on. You can pick up side quests with your 2nd, 3rd, 4th class if you like, but I wouldn't recommend it because it's just a pain in the butt to run from A to B to Z for no reason. It will also take magnificently longer if you're switching between the classes and trying to level them that way.
Keep doing that till you're level 17 I think it is, or after you've have finished the first 3 dungeons. Go unlock palace of the dead (quest starts in gridania), but ignore it after you've unlocked it. If you're new to the game, I wouldn't recommend jumping right into that because you're missing out on a lot of great story. But when your main character reaches level 30 you want to join palace of the dead with your other characters, since you will need the cross class skills, or you can just continue doing the side quests but it will take longer.
After you've gotten all your characters that you want to play, or that you need for your cross classes to 15/34, you can continue with your main character (level 30+) on the main scenario quests. From here on out, there's no reason to do the side quests anymore, as mentioned all the msq's have been buffed and you'll find yourself out leveling the content faster than it has been prior.
Doing the side quests will just make it worse and up'ing that by at least 8 - 12 levels. Than you run in the problem that you've to buy gear (unless you learned crafting)etc. You can get from level 30 to 50 easily with just doing main scenario, dungeons for the main scenario quests and unlocks for the grand companies.
If you have the time or are willing to put the time into it, there's a couple of zones, that will be great exp for your main character with doing both, main quest as well as side quest. The sylph area down in gridania is one of them, little ala mhigo, coerthas highlands. But again, that will add at least 2-4 hours to your game time, if not longer and it will add a lot of levels to your character. You might end up level 47+ before even setting foot into mor dhona. So keep that in mind.
Hope that novel helped, if not just ignore itThat's by the way the method for ARR content, HW will look totally different but we're getting to that another time, enjoy leveling to 50 first.