If you just started a game, by now you may already have a better idea of what you want and how the game works from the initial combat encounters. If not:
The companions you can pick up early in the game include a 2 handed warrior, water/air mage, bow ranger and dual dagger wielding scoundrel, though you can develop them however you wish as they level. A party can easily have multiple mages, or a couple warriors or archers (I wouldn't have then use the same weapon type, though), but I don't know how having more than one rogue would work (have not seen anyone report doing that).
A balanced party works best, having a variety of melee/magic/ranged attacks/spells/skills to be able to adapt to different situations. Of course some overlap is fine (especially once you are familiar with the game), and you may want some or all character to know some healing or boost spells (like Oath of Desecration), or have a ranger learn the Teleportation spell, etc.
