>>1 to me, religion is a very personal belief, I'm agnostic and I have my own personal beliefs about life but I don't preach them to anyone. It's okay to have a religion different from that of your parents, however, if they're the type who might hold it against you or be really saddened by it, then maybe just keep it to yourself until you're older. There isn't really anything wrong with that if you want to still have a nice relationship with your family.
However, if you think they'll understand, maybe not fully, but at least your reasoning behind it, then tell them. It maybe hard but I believe that you should go for it for the sake of just letting it become official and off of you chest.
I hope this helps. ^^