It's is a contraction of it is or it has.
The apostrophe in a contraction such as it's means that two words have been combined, and some letters have been left out.
For example: isn't = is + not
they're = they + are
don't = does + not
It's combines it is or it has and leaves out letters:
it's = it + is
it's = it + has
Use it's only when you mean it is or it has.
Example of it's meaning "it is"
Example of it's meaning "it has"
Its is the possessive form of the personal pronoun it.
Like the other personal pronouns, it forms its possessive without adding an apostrophe.
Examples
Do NOT put an apostrophe in its to make the word possessive!
REVIEW:
it's = it is or it has
BUT
its = possessive form of it, meaning "belonging to it"
Link to exercises on t's / its.