Its vs. It's

Its refers to the last single noun referred to.

• Example: At the dog track today, the dog I bet on won. Its come from behind victory over the other dogs was remarkable.
  

It's is a contraction of it and is.

• Example: It's about time I made to the next level in the game.

Memorization tip: When in doubt, say the sentence using "it is." If the sentence makes sense, use it's. If the sentence does not make sense, use its.