IWD1 has a much more simple system. I'll summarize the basics -

Fighters, Paladins and Rangers are your melee and/or tanks. Give them high strength, mid dexterity unless you plan on going archer(I think you may as well multi or dual class w/thief for an archer though), high constitution, and decent wisdom for will saving and for the ranger and paladins' small spell selection. For paladins you'll also want good charisma(increases their saves, meaning they resist spells and such more effectively) but it's automatically fairly high so.

Paladins will be particularly good against spells with high saving throws and have some extra abilities and small amount of cleric spells, while Fighters will put out more raw damage since they can specialize further in weapons. Rangers can stealth and get some druid spells and tend to have pretty high stat rolls - though if you reroll enough any class can.

Clerics and Druids are the main divine casters. Wisdom is your main stat for them. Clerics can wear heavier armor and thus will need less dexterity and can be a bit more tanky/melee based. They have more healing and defensive oriented spells, and can also turn - frighten or destroy(or if evil control) - undead. Druids have a more offensive spell set and can shapeshift to melee but it's of limited usefulness.

Thieves take care of locks and traps and so on, you'll want good dexterity since they can't wear heavier armors, and some intelligence to boost your thieving skills. They are fairly weak in combat but when attacking from behind from stealth deal increased damage and are decent for giving caster enemies trouble.

Bards are a jack of all trades sort, some arcane casting, some (negligible, IMO) thieving skills namely pickpockets, and songs. The latter is what they're really useful for since they give significant bonuses to your whole party, and also identifying items although mages can do this as well with a spell. Light armored so best used as archers when using weapons. Bards will mainly need charisma, dexterity, and some intelligence.

Mages are the arcane casters, intelligence is their main stat but they also can't cast in armor so you'll want good dexterity too. They have a number of utility and control spells, buffs, and of course fireballs and such for damage.

It's worth noting that constitution is an important stat all around as it gives your characters additional hit points which are fairly nice for keeping everyone alive. Another thing worth noting is that some classes really don't need some attributes, and you can drop them as low as possible(to put more points in more helpful attributes) with no loss. These stats are intelligence(only mages, thieves really need) and charisma(fighters, rangers, thieves, wizards get no significant benefit from, clerics don't need it much either). It's considered cheesy, or "power gaming" IE not role-playing spirited to do so by some, but to each his own.

Then there's multiclassing and dual classing. Multi you level as both classes, dual, which is humans only, you start as one class and switch to another. Until you reach the level of your first class with your second though, it is deactivated, so dual classes go through a period of weakness that is made up for later. I find the best class to dual or multi with is fighter, especially for thieves who benefit a lot from the additional combat capabilities.
When dual classing it's best to always take non-caster classes first, as otherwise you cut off the spellcasting classes' spell progression too early.
Also worth noting that you can take a party of fewer than six, which is where multi-classes really shine.

The core of a good party, IMO, is -

1-2 Fighter/Paladin/Ranger
1 Pure Cleric or Fighter dual classed to Cleric
1 additional Cleric or a Druid, possibly multiclassed with fighter
1 Thief, ideally multi or dual class with fighter or mage

The last 1-2 are entirely personal preference - some choices may be more powerful but with the above you have all you need anyway. I'd recommend a mage or fighter/mage just to get haste though, if you didn't make your thief a dual or multi w/mage.


An example party -

Fighter
18 Strength
12 Dexterity
18 Constitution
8 Intelligence
12 Wisdom
8 Charisma

Paladin
16 Str
12 Dex
16 Con
8 Int
14 Wis
17 Cha

Cleric
14 Str
12 Dex
16 Con
10 Int
18 Wis
10 Cha

Druid
12 Str
14 Dex
14 Con
10 Int
18 Wis
16 Charisma

Fighter/Thief
14 Strength
18 Dexterity
14 Constitution
14 Intelligence
10 Wisdom
12 Charisma

Mage
10 Strength
16 Dexterity
16 Constitution
18 Intelligence
8 Wisdom
10 Charisma

I might summarize IWD2's system later but it's much more complicated, I'd recommend playing IWD1 first to get introduced to the more basic D&D style combat.