I don't know how to tell my grandparents and parents i'm atheist.?

Okay, my family firmly are christians. My grandparents, Are totally beleivers. They get angry if i say "Oh my god." Like i said a really bad cuss word or something, I'm a 14 year old boy by the way. I just don't beleive in god, There's no proof! I think the bible is a load of crap, And if you don't beleive in god you go to the suppose hell? It's just not beleivble, That a guy with a beard, Has all magicall powers and is sitting up there on the supposed "Heaven." Can someone help me, Try and help me explain this to my grandparents, Because i really want to tell them i'm atheist, But i don't know how.