it’s is the short form of it is or it has (been) :
- When you go abroad, it’s a good idea to take a guidebook with you.(… it is a good idea…)
- Don’t worry, it’s been like this for a long time.(… it has been like this…)
- Are you telling me that it’s my fault?(… it is my fault?)
- The box was hidden in an attic during the war and it’s been there until today.(… it has been there…)
Its is a possessive word, just like my or your (something owns something or something belongs to it).
- The car was badly damaged but its driver escaped unhurt.
- The dog went back to its house.
- The house and its garden are a popular tourist attraction.