A character gets a certain number of skill 'ranks' per level, which you can place in different skills. The number of ranks you get to place are based on your ability scores. You get a number of ranks in dexterity-based skills every level equal to your Dexterity score, a number of strength-based skills every level equal to your Strength score, etc.. So, an average human (who has 3 in every ability) gets three dexterity based skills, three strength skills, three endurance skills, three intelligence skills, and three spirit skills every level. This means that almost without exception, every character will get 15 skill ranks per level, split up amongst the different abilities
For most skills, every rank you have in a skill boosts your skill modifier in that skill by +1. Your skill modifier is what you use to make skill checks so if you are making an Acrobatics check to jump two metres (with a DC of 6), and you've placed two ranks in Acrobatics (giving you a modifier of +2) when you make your check you get a +2 bonus to your die roll. A character with no ranks would have to roll a 6, whereas you, with your two ranks only need to roll a 4 (because 4 + 2 = 6, the DC).
Some skills have a …staggered‚ skill rate. This means that as you put ranks in, you get techniques and an increase in the skill bonus alternately. The first rank you put in gives you a technique, the second increases your modifier by one, the third grants you another technique, etc.. The end result is that at every odd-numbered rank you can select a technique and every even-numbered grants a +1 modifier. A staggered skill represents one in which learning is either slow, or which is so chancy that a professional (that is, a character with high ranks) does not have as big of an advantage over a novice compared to most skills. The majority of combat-related skills, and skills in which opposed checks are frequently made, are staggered.
You may not have more ranks in any one skill than your level.
Only your base abilities determine skills per level, not ability boosts (even from permanent racial bonuses), drains, or temporary modifications.