Warriors should focus heavily on Strength, with Stamina and possibly Agility as secondary stats.
Assassins should focus almost solely on Agility, with Stamina as a secondary stat if you feel you need more survivability/HP.
Mages should focus on Intelligence, with Stamina as secondary.
When I say secondary, I mean that you should put all or most of your stat points into thhe primary stat, and then use gear that gives the secondary stat or use only 1-2 of the stat points on them.
You can find some decent guides for each class just by searching Google for something like "Dragon's Call Warrior Guide" or something along those lines.