Ah, a man after my own heart!
Apostrophes are used in contractions (it's) and to show possession (but not in "its," the possessive form of the neuter singular pronoun). They are used to show plural forms only in very rare situations - and you can usually leave apostrophes out even in those situations.
When in doubt, leave it out.
Joe Offer, Self-Proclaimed President
Commission to Stamp Out Extraneous Apostrophes
(that's what I get for being the son of an English teacher)