lvl

1-10 Grind DoW and DoM classes, lvl DoH through low lvl Levequests at the Adventurers Guild, Grind DoL

10-20 Start hunting down class quests in their respective cities - every city is a host to a number of Dow/DoM/DoL/DoH classes where you can obtain class specfic quests; Gathering will fuel your Crafting classes; lvling your DoH classes will help you gear yourself out on the cheap; Guildleves are great for solo.

20-30 Find yourself a ls if you haven't by now, you might want to try partying for your DoM/DoW classes; The same holds true for gathering and crafting classes - there is a synergy between the two of them; as for levequests if you do the lvl 30 Fieldcraft quests available at lvl 25 to gathering classes, the levels fly by. If you can find a ls member nice enough to help you kill the various world NMs for gear and achievement you could gear yourself out nicely at this point. And then there is chocobo and Grand Companies...

30-40 Party for DoW/DoM classes; Grind DoL classes from 30-35 then do lvl 40 Fieldcraft Leves from 35-40; Use the items you gather to grind your DoH classes; Check out what Grand Company has to offer; and Dont forget to do Class Quests as they become available as well as main storyline quests, etc.

40-50 Grind Grind Grind DoW/DoM on Raptors and Beastmen Strongholds; farm Grand Company Seals; gather goodies and do levequests for your DoL; then use those items to lvl up yer DoH.

50+ Primal Ifrit, Dark Moogle, Darkhold, Beastmen Strongholds, 2+meld Materia, Achievements, More to come etc...