When should I use an apostrophe to show possession?

Apostrophes show possession. To show possession with a singular noun, you simply add an apostrophe plus the letter 's.'

The girl's dog walked quietly by her side.

The boss's wife showed up unexpectedly.

Mr. Smith's daughter left for university.

Be careful not to add apostrophes to plural nouns that don't show ownership.

Apple's are $0.25 each.

The above example is wrong because "apples" in this sentence is plural; it doesn't show ownership of anything. It should just be:

Apples are $0.25 each.

Another very common mistake is to add an apostrophe to the pronoun 'it' when showing possession.

The dog licked it's leg.

In this instance, you don't add an apostrophe because 'it's' is the contraction for 'it is.' The possessive form is 'its':

The dog licked its leg.