Choose one type of weapon. You can also choose unarmed strike or grapple (or ray, if you are a spellcaster) as your weapon for the purposes of this feat.
Proficiency with selected weapon, attack bonus +1.
You gain a +1 bonus on all attack rolls you make using the selected weapon.
You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new type of weapon.
D20 Pathfinder SRD Weapon Focus Article- http://www.d20pfsrd.com/feats/combat-feats/weapon-focus-combat---final