On Mario 64 How do you get all the stars

There is no cheat code for that, but I can give you some tips:
1. Grab coins. You can get a star for getting 100 coins in each level (only the main courses, not secret areas or bowser levels)
2. Look around the castle. There are 15 stars inside the castle and on the castle grounds. The Bowser levels each have 1 star (Red Coins) which are listed with the secret stars.