Writing Resources: Apostrophe and Possession
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To indicate possession

When something belongs to someone or something, show the relationship between them by using an apostrophe and an ending s. For example, if the ball belongs to Jim, write:
That is Jim's ball.

If the person or object ends in s, adding an additional s after the apostrophe is optional. A good rule of thumb is that singular nouns take an s while plural nouns do not.
The census's results are inconclusive.
The books' bindings are in need of repair.

Most common pronouns, however, never use apostrophes to indicate possession:
That is his ball. I will not sacrifice its life.
That is his' ball. I will not sacrifice it's life.

The possessive forms of common pronouns are as follows. Notice that none of them use apostrophes. The difference between its and it's is a frequent source of confusion for students. Its is possessive; it's is a contraction meaning it is or it has.
Pronoun Example
my, mine

My shoe is missing.
your, yours May I wear yours?
his Why not wear his shoe?
her, hers It looks too much like hers.
its What happened to its sole?
our, ours It is not ours.
their, theirs It is theirs, so I returned it to them.

In contractions

Contractions are shortened versions of common expressions. For example:
I am hungry I'm hungry

The important thing to remember about contractions is that the apostrophe takes the place of the missing letter(s), so it goes where the letters would be--not at the end of the word.
He's strange. It's an unlikely occurrence.
Hes' strange. Its an unlikely occurrence.
Its' an unlikely occurrence.

Its' is not a word. The missing letter in the contraction is the i in is, so the apostrophe must precede the s. And remember, if it is used as a possessive, there is no apostrophe.