It's or Its

"It's" is contraction of "it is" or "it has".
Examples:

• It's (It is) a plane!
• It's (It is) my bed time.
• It's (It has) been a long time.
• It's (It has) been good to know you.
• It's (It has) got to happen soon.

"Its" is possessive form of it (belonging to it).
Examples:

• The bird lost some of its feathers.
• Where is its head office?