You can’t 100% believe in no reality. Your mind will eventually reject the proposition. Reality is the solidity of things beyond your ideas about them. Even if you tell yourself you don’t believe in it, eventually it just becomes an exercise in lying to yourself. That doesn’t mean it isn’t useful, though!
What you can do is to chip away at the perception of the solidity of reality. One way to do this is to walk around for a few days to a week and announce to yourself, about everything you see, hear, touch, and otherwise perceive, that this is not real.
Your computer is not real, outside is not real, inside isn’t real either. Your beliefs aren’t real. You aren’t real. The school you go to isn’t real, neither are the kids in it. The lessons aren’t real. The bike you ride on to get there isn’t real either.
You’ll forget for awhile but just gently return to denying the reality of everything the second you remember. Eventually things start to feel really strange and your mind will ‘reboot’. Great thing to do when you’re younger to relieve existential angst.