Essentially you want a balanced party.
A mage is virtually a pre-requisite (unless you're a mage yourself) as they have heal. Mage is like your buffing class, they don't have much in the way of attack magics.
Sorcerer's are your heavy hitting magic class with ranged AoE - this means long cast times but really powerful. I don't recall if they can buff you or not, but they definitely don't have heal.
Fighter is your tank class, not much to say beyond that. Single target damage and aggro skills.
Warrior is your 2 hander - slow heavy hitting melee, can attack groups of mobs. Swinging a giant claymore is always fun ^_^
Strider is the rogue-like. Use daggers and bows to a medium level - strider itself is pretty pointless once the advanced classes unlock.
Ranger - Class advancement on strider, focuses on bow techniques. So, ranged melee class.
Assassin - Other advancement on strider, focuses on daggers. Bursty single target dps.
Then there is the magic hybrids that only the arisen can be. Magic archer and magic knight. I've avoided using these as they're OP and will simply ruin my game experience.
As for your party make up, it depends on what you're playing as - and what your playstyle is like -
and what you're fighting -
and how far in the game you are. My party will be different for an Ur-Dragon fight than for a long dungeon crawl.
Early game, Fighter - Mage - Strider - other DPS (Strider, Fighter, Warrior, Sorc, Ranger, Assassin). From there you'll find what works for you
Pro tip: Don't just look at the pawn's class, look what skills they have equipped. Some people are 'tards and will only have 1 skill on their mage. Ingle is heal I think? Might be Anodyne.... I forget the spell names. Fire/Ice/Holy etc Boon's are your buffs - I recommend at least Fire or Holy for most things.
Also, hired pawns do not level with you - so every 5-10 levels change your pawns.