Apostrophes are used in two ways: to show possession and to show the omission of letters.
To show possession
To see if you have a possessive, turn the phrase around and make it an "of the…" phrase.
the boy's hat = the hat of the boy
To place the apostrophe correctly to show possession
To show omission of letters
In contractions one or more letters (or numbers) have been omitted, and the apostrophe shows this omission. Contractions are common in speaking, dialogue, and informal writing.
I'm = I am
he'll = he will
don't = do not
'60s = 1960s
who's = who is
can't = cannot
Do not use apostrophes for possessive pronouns like his, her, its, yours, and ours.
Do not use apostrophes to make nouns plural.
Compiled and edited from Purdue Online Writing Lab (OWL), http://owl.english.purdue.edu