Here's a simple class guide:
Job Classes and Requirments

Single Class Jobs
Squire:N/A
Chemist:N/A
Knight:Lv.2 Squire
Archer:Lv.2 Squire
Monk:Lv.2 Knight
Priest:Lv.2 Chemist
Wizard:Lv.2 Chemist
Time Mage:Lv.2 Wizard
Summoner:Lv.2 Time Mage
Thief:Lv.2 Archer
Oracle:Lv.2 Priest
Meditator:Lv.2 Oracle
Geomencer:Lv.3 Monk
Lancer:Lv.3 Thief

Multi Class Jobs
Samurai:Lv.3 Knight, Lv.4 Monk, Lv.2 Lancer
Ninja:Lv.3 Archer, Lv.4 Thief, Lv.2 Geomencer
Calculator:Lv.4 Priest, Lv.4 Wizard, Lv.3 Time Mage, Lv.3 Oracle
Dancer:LV.4 Geomencer, Lv.4 Lancer(girl only)
Bard:Lv.4 Summoner, Lv.4 Meditator(guy only)
Mime:Lv.8 Squire, Lv.8 Chemist, Lv.4 Summoner, Lv.4 Meditator, Lv.4 Geomencer

Grind, grind, grind to get you characters to the class you want them to be Also EXPERIMENT! Some of those classes aren't very powerful, but have great unique abilities that might just come in handy