Many people get confused about apostrophes.
They are used for two main reasons: contraction and possession.
1) Contraction (sometimes called omission) is the term used when words are shortened by missing out letters.
can not / can't
I am / I'm
she is / she's
2) Possession means ownership and this is the area that a lot of people have trouble with.
The apostrophe comes before the 's' if there is only only one person involved.
the boy's toys
If there is more than one boy then the apostrophe comes after the 's'.
the boys' toys
Some plural nouns don't end in 's' so the apostrophe comes after the noun but before the 's'.
the children's toys
Many people put an apostrophe when they make a singular noun into a plural noun but this is not correct.
Cream tea's £3.50. (Incorrect)