With each skill point granted by your Expert levels, you can buy 1 rank (equal to a +1 bonus on checks with that skill) in any one skill. Other classes (not yet in play) may grant skill points too; ranks bought from different classes stack.
You have a maximum rank in each skill, beyond which you cannot buy ranks. The maximum rank is determined by your classes and levels. For Expert levels, your maximum rank in any skill is 1½ times your Expert level (round halves up). Levels in certain other classes not yet in play may increase your maximum rank in some or all skills - for each skill, maximum ranks from all applicable classes stack.
To make a skill check, roll: 1d20 + skill modifier (Skill modifier = skill rank + ability modifier + miscellaneous modifiers)
This roll works just like an attack roll or a saving throw - the higher the roll, the better. Either you're trying to match or exceed a certain Difficulty Class (DC), or you're trying to beat another character's check result.
A character's number of ranks in a skill is based on how many skill points a character has invested in a skill. Many skills can be used even if the character has no ranks in them; doing this is called making an untrained skill check.
Each skill is associated with a key ability; the skill check is modified by the key ability modifier. The key ability of each skill is noted in its description.
Miscellaneous modifiers include armor check penalties and bonuses provided by feats, among others.