How to tell my parents that I don't believe in Catholocism?

Your best bet is to have an honest conversation with them....but if you feel that is not an option, you may just have to wait 2 years until you are old enough to make that decision without having to deal with the fallout. I am Wiccan and was raised Baptist. I understand you completely! Go through the motions, but believe what you will in your own heart....no one can take that away from you.