At every even level, your character will receive a +1 enhancement bonus to an ability score of your choice. You cannot choose to apply this bonus to the same ability score twice in a row. Ability score raises that come every 4 levels are unchanged from core rules, and therefore are not considered enhancement bonuses. This means that a character could raise his Strength at 4th level per the normal rules, and apply a +1 enhancement bonus to Strength at the same level, for example.
Every level (including first) you can choose from one of the following six categories: melee, ranged, armour, shield, spells, and saves. You will get a +1 bonus to that category every time it is chosen. You can choose any one of the six once every block of 4 levels (1-4, 5-8, 9-12, 13-16, 17-20). So you cannot choose +1 to saves at 3rd and 4th level, but can choose them 4th and 5th level.) And yes, this starts at 1st level.
The six bonus types work as follows:
Melee (enhancement): +1 to attack and damage with manufactured melee weapons, natural attacks, and unarmed strikes. +1 to CMB checks and to CMD. +1 to attack (but not damage) to melee touch attacks (but not those used as part of a spell). For purposes of bypassing damage reduction, any weapon you wield counts as a magical weapon with a straight bonus equal to your melee bonus – 1 (e.g, when you reach a +4 melee bonus, your melee weapons count as +3 (magical) and silver for purposes of overcoming damage reduction, damaging incorporeal creatures, etc.).
Ranged (enhancement): +1 to attack and damage with manufactured ranged weapons. +1 to attack (but not damage) with splash weapons. +1 to attack (but not damage) to range touch attacks (but not those used as part of a spell). For purpose of bypassing damage reduction, these bonuses work the same as the melee bonuses indicated above.
Armour (enhancement): +2 bonus to AC (regular AC, touch AC, and flat-footed AC). You do not have to wear or use armour to receive this bonus.
Shield (enhancement): +2 bonus to AC (regular AC only, not touch AC or flat-footed AC), but only if you are wielding or wearing a shield (or a special weapon like a klarr which provides a shield bonus). A shield spell doesn’t count. Not all characters will make use of this bonus.
Spells (enhancement): +1 to attack and damage with spells. For spells that target multiple creatures independently (magic missile, scorching ray) the damage bonus applies once per spell to one target of the caster’s choice. For spells that affect multiple creatures simultaneously (fireball) the damage bonus applies to all. The damage bonus only applies to spells which deal hit point damage, not ability damage, or spells which do not deal damage at all. Cure spells used to heal are not affected by this bonus (though the attack bonus would apply if it were required), but cure spells used to harm undead creatures would be affected.
Saves (resistance): +1 bonus to all saving throws