Apostrophes have three main uses:

1. to take the place of deleted letters as in
that is = that's
have not = haven't
it is = it's
cannot = can't

2. to show possession—to show that something belongs to someone. Use an apostrophe with an "s"
summer's song
Isaac's dog
children's toys
the Joneses' new land rover

3. to show the plurals of letters
cross your t's and dot your i's
there were five renaissance's misspelled in his report

Note# 1: Do not use apostrophes with personal pronouns to show ownership:

mine
yours
his
hers
theirs
ours

Do not use an "apostrophe - s" to show possession with "it": Its fangs were terrifying. The word "it's" is a contraction for it is: It's time for people to step forward and volunteer. (If you get confused with "its" and "it's," simply replace the word with "it is" to determine whether you need to use an apostrophe.)

Likewise, watch out for confusion between these three other commonly confused sets of contractions and possessives:

your/you're

their/they're

whose/who's

Your idea was integral to the project. You're the best consultant in the area.
Their lobbying efforts paid off. They're noncommital about the vote.
The men whose jobs were lost were compensated. The woman who's running for senate will probably win.

Note # 2: MLA style does not use apostrophes to form plurals of abbreviations or numbers:
RNs
4s
1990s

Exercise 1: Apostrophes