How do I tell my parents that I don't fully believe in Catholicism?

I suppose it would depend on how conservative your parents are. I find it's easier to wait until you are out on your own because you won't be around them every single day and it will be easier to get some space that you'll need once you tell them. I personally waited until I was out of high school because I have really conservative parents but however and whenever you do it, good luck.