As with most RPGs, character creation involves the first crucial decisions you have to make. Divinity: Original Sin is both old-school and new-school in how it approaches character builds. You start by choosing from preset classes like "Wizard" and "Ranger," but those are actually just suggestions that in no way restrict the abilities and skills you can have. Therefore, it's a good idea to pick a "class" as a template and then slightly re-spec it to be as potent as possible. You then chose which stats (Str, Dex, Int, etc.) to upgrade, which abilities (bows, magic schools, sneaking) to invest in and which talents (persistent traits like perks in Fallout) to assign.
The important thing is that you not assign the same abilities to both main characters. Maximum party size is 4 - this is reached with 2 main characters and 2 recruited companions.
It's worth keeping your primary starting party in mind when building your main characters. In the first town, you'll have the option to recruit up to two companions from the following list: a two-handed warrior, a hydro/aero mage, a ranger, and a rogue. So your party composition will have many options.
It can be advantageous to have one character with at least one point in the four main magical food groups: Aerotheurge (air/electricity), Geomancer (earth/poison/protection), Hydrosophist (water/ice) and Pyrokinetic (fire). It's worth bearing in mind that the primary magic-user NPC who joins your party specializes in air and water, and also that fire and earth work well together, so if you want to get more specific, it's smart to plan a focus in those latter two abilities. But to start with, if you have one point in all four, you can learn any spell, it'll just cost more action points to cast some of them. That's useful in case you specialize in fire but find yourself fighting demons who are immune to fire—you don't wanna only have fireballs!