Your level of Tactics determines how much Base Damage you are capable of inflicting on opponents. It is a necessity for any melee fighter or archer.
Tactics is used during melee and ranged combat, whereas Evaluating Intelligence is used to support magic skills.
Tactics provides a damage multiplier to your base damage along the following equation:
Tactics Damage Modifier = (Tactics - 50) / 100
(Temporary note on above formula: the actual damage difference between 0 and 100 Tactics is about 400%. The above formula is misleading/incorrect. Tactics is absolutely required to deal significant damage as a melee/ranged fighter.)
Tactics is only gained on a successful hit, not on misses. Therefore, to quickly gain tactics, fight mobs of equal or below your weapon skill level to increase the amount of successful hits.
An easy method is to grab a training weapon (or buy wrestling up to 40 from a Mage NPC) and hit animals or low-damage mobs like Headless (in Covetous) or skeletons/zombies.