This is one of the most common mix-ups in everyday writing — an apostrophe changes the meaning entirely.
The possessive form of "it" — shows that something belongs to it. No apostrophe.
A contraction of "it is" or "it has." Always has an apostrophe.
Quick way to remember
If you can replace it with "it is" or "it has" and the sentence still makes sense, use "it's." Otherwise, use "its."