There are a couple of simple ways to distinguish it’s from its:
- It’s is a contraction. It stands for it is. Whenever you wish to say it is informally use it’s as in “It’s almost time for lunch.”
- Its is the neuter possessive. It performs the same function as his, hers, and ours. Where you want to indicate possession use its as in “After the tornado, the house was still sanding but its roof was missing.” The missing roof belongs to the house. Use its as you would use his or hers: “The backpack is his” or “The purse is hers.” Notice that there are no apostrophes in its, his, hers, yours, ours, or theirs.
One more thing: there is no such word as its’ but I have seen it recently. This is a case where a writer thinks that there must an apostrophe somewhere so he adds it to the end just to be safe.
The difference is between a contraction (it’s) and possession (its). If you remember that distinction should have no difficulty in using the correct word.