It's means it is.
It's a beautiful day. [It is a butiful day.]
Its means belongs to it. The dog scratched its ear. [the dog scratched the ear that belongs to it.]
An easy test is to substitute the words "it is" in the place of the its you want to use, and see if the sentence makes sense. If it does, use it's. If you're still not sure, substitute the word "his" in placde of the its. If it makes sense, use its.