Attacking works as thus:

Attacker rolls a 1d20. Every attack has an accuracy check, or AC. Defender takes attacks AC and their approrpiate Evasion bonus (speed, defense, or special defense) and adds it to the AC. If the d20 roll is higher than that, the attack hits. Damage is calculated by whatever base damage the attack has, which is on a chart in the book. Base damage plus appropriate stat (attack or special attack) is raw damage. Defense or special defense is applied, then super effectiveness or resistances are applied to that resulting number.