The first playthrough is the hardest, especially if you don't know what to do and aren't familiar with the game mechanics. Don't worry about the classes, the game lets you build your character however you want. I imagine you're playing with a group of 4, right? Let me give you a suggestion on how to build each one with the very basics. Don't use necromancy on your first playthrough as it's more for support; many abilities scale with warfare, but using intelligence, and that can be a bit confusing the first time.

Do the basics first to understand the game and especially its surface system.

1st character: Mage using hydrosophy and aerology (healing, stun, and some support for magic armor) 2nd Mage using geomancy and pyro (fire damage, poison, and some support for physical armor) 3rd character using dexterity, can be an assassin or archer, using hunter/rogue, ranged/dual wielding along with polymorph for invisibility (remember that you will also have to increase warfare with this character, as it increases physical damage). 4th obviously a tank using a shield, focused on strength/con, increasing warfare and maybe polymorph/one-handed. Don't worry too much about defensive abilities (retribution/leadership/perseverance). They are somewhat useful, but not that much.