How can I tell my parents that I don't want to follow their religion anymore?

Since your mind is made up, then you know that what they say won't sway your decision. Keep in mind, they have tried to raise you the "right" way, or what they consider to be right. I guess that out of respect to them, you could keep silent until you are old enough and move out on your own. But if you think you are strong enough to handle their reaction, then tell them sooner rather than later. It really depends on you. Even if they were to say how much they have spent educating you, giving you the Bar Mitzvah, etc., you know your mind is made up. The question to ask yourself is "when do I want to face them with this?" Anyways, when you are old enough to be on your own you'll be able to practice whichever faith you choose, and it won't be their business, though they may not accept it and they may be concerned/ angry/ saddened- in the end, the choice is yours.