There are a few simple rules to follow when using apostrophes:
To show possession, add an apostrophe + s to the end of the word, even if the word ends in s. For example: The dog's toys are under the bed.
To create a contraction, or shortened version of a word, place an apostrophe where the letter or letters have been removed. For example: I'm going to the store.
To indicate that a letter or letters have been omitted, use an apostrophe. For example: rock 'n' roll
If you're ever unsure of where to place the apostrophe, try reading the sentence out loud. If it sounds correct, then it probably is!