If you haven't already received an answer, you can relax! Sub job quests and low level gear-requirements are more forgiving than ever.
In short, you go like this:
solo to 12
go to gusgen mines
join alliance and fight skelletons
leave at 30 and solo flies and crabs
turn in sub job quest
level another job to 30
crawlers nest GoV pages
level limit break
more crawlers nest
Long version... go like this:
Levels 1-30 were always a wash, and most of us long-term players would solo 1-15 before we even worried about equipping a piece of gear by speed-killing easy prey / decent challenge enemies (it was still more efficient, even when they were ~30xp a kill). Today, you can do that much faster since each kill on an easy prey awards over 100 xp and at those levels you have less than 2000 xp per level (so really not many kills at all). You also get tabs from field of valor books which gives refresh/regen for cheap, which is amazing for low level starter characters.
The way things are now, is those field of valor books have slightly different versions inside the "dungeon" zones called Grounds of Valor.
The way the community seems to do it right now, is everyone piles up in Gusgen mines (a cave-zone near bastok, you can walk to the camp easily from Konschtat highlands teleport dem). They join up in an alliance and farm skelletons to gain piddly XP. The idea is that you only need to 'receive xp' for a skelleton kill to get credit on the Grounds of Valor book quests, so you get very little xp per kill (like 3xp sometimes) but you get 1000~ish every time you complete a page. Since there are so many of you, you can grind through the skeletons from the GoV page-1 very quickly. Most people go there around level 12, so I'd recommend soloing to at least 12 before you head out.
By the time you are done leveling in Gusgen Mines, there's a very good chance you'll find the hardest Sub-Job quest item; "A Magicked Skull" (they drop from the ghouls you will be fighting repeatedly). If you don't have one by the time you are ready to leave, Stick around until you DO get one. Ask if you can lot on one if it drops, most people won't mind because it doesn't hold any other real value except for the higher level limit break quest.
Try to stick around gusgen and re-sync until you are late 20s if possible (most people decide to leave there at level 30). Lets just assume you make it to 30... at that point you can step outside, go kill some flies and crabs in valkurm dunes that will undoubtedly be easy prey to you, so you can farm the remaining items for your subjob quest. Then just go turn in, unlock your sub job, and do the leveling in gusgen thing all over.
Now you should soon have a level 30main/15sub character (if you take both jobs all the way to 30 it will cover whichever one wants to level as main all the way to 60 without having to relevel your sub, and is generally a good idea).
Do some relaxing, explore a bit, think about moving to 'the big city' (jeuno) and take some time to gear up. Before now, gear was terrible, but around level 30 some better options open up, in particular, you'll want a good weapon if you are a melee, and to consider the aquisition of some of the gear designed for CoP, as the stats tend to be pretty great for low level gear. As a mage, look at setting aside some money for a dark-staff and whatever-other-elemental-staves you think you can afford (if you are poor, dark and ice will get you buy for a black mage for a long-long-time, the same goes for dark and light staves for a healer) These level 51 staves heavily benefit you from level 51, ALL the way to level 99. They aren't really 'optional' for a low level player with no cash. If you can't afford at least a couple of those, you can still grind through the GoV pages, but you'll want to stop and go farm gil once you feel you are high enough to start working towards those items. One good way to make money as a starter, is to use the points you aquire from wearing Signet and killing mobs that give you xp, to buy items that sell. One very popular item right now is "void dust" that you can buy from one of the voidwatch NPCs, I believe they sell for like 30,000 gil and cost about 2000 points. A few of those and you'll be geared well enough to reach 78 in no time (at 78, gear aquisition gets a little easier, because there are very cheap, very obtainable Abyssea gear sets that are VERY good stat wise compared to most gear in those level ranges). Another thing that would be good to do at level 30 is start the Abyssea questline, so you can begin accumulating stones that are used to access the zone (they accumulate over time, so the sooner you start the clock the more time you'll have when you are high enough to enter).
Head to crawlers nest to do the GoV pages there vs crawlers and bees. That page tends to take people all the way to 60 more often than not, so you can level to your cap in the 50s, and then go do your level-cap unlocking quest. You might meet some people in your xp party that are willing to help you, but if not, you'll probably want to shout in Jeuno or join a social linkshell with people kind enough to /assist you. An early social LS is a good idea, because while they won't make you super rich most likely, and won't be able to help take down the strongest-of-all enemies most likely, they will probably be the people you build memories and strong bonds with, and over time, you'll probably accomplish a lot more with a good social linkshell than you will with a big "Voidwatch" or "Legion" linkshell. You'll want to take time to get khazam keys, the airship pass (do missions through rank 5 I believe, been a long time), and possibly start working on important Artifact gear piece acquisition.
From here it gets a little blurry, level 60+ is still annoying as balls to me, they say you can go to the sewers under Sandoria (you get to them from inside the castle) but when I went, there wasn't really anyone there so I ended up going about xp in a roundabout way.
The only thing that really bothered me about this new leveling style is the lack of variation. I would have loved to see more parties pushing for a Mandragora in Y. Jungle party (that's something I always looked forward to when leveling, the relaxing feel of Kazham). And in the 60s, I think it would be fun to go to places like Sky or Boyahda tree, but I see why those places are not popular (far to reach, other people are already in those zones for other reasons potentially screwing up mob spawn timing, and in general, lack-of-access for new players without a lot of work). I guess I'd rather level-sync down to Y. Jungle fights than go to Sandy sewers... being stuck in those ultra-confined, ultra-dark spaces just doesn't thrill me.
Posting from just above the generator.