Evo said:
A true atheist doesn't care about religion, they just dismiss it as nonsense. They don't sit around thinking about it. The "atheist" websites where people can go and talk about it, I think is ridiculous, except in the case of the OP where it wouldn't be to talk about being atheist but perhaps as a means of talking about people in your life that go crazy when you tell them you don't buy into religion anymore.
A true atheist doesn't care about god, but they can and do care about religion, since so many people make their personal/political choices based on religion, so it does have a very real impact on atheists' lives. Most "atheist" websites, are devoted to the http://www.talkorigins.org/" . They also serve (as you point out) for people to have a place to vent about religions impacts on their own life (I don't really understand how you say they're "ridiculous", and then go on to say "but they serve this one purpose").
Another goal of the "new atheist" movement is to let others who reject religion know that they're not alone, and that they don't have to be afraid to speak up. For many people (I'm thinking JWs, Mormons, and similar), while they do go out and proselytize frequently, they are actively discouraged from socializing outside their religion. This means they have no friends, family, or in many cases (home-schooled children) even casual aquaintances outside of their religion. Outspoken atheists give people like this another option, let them know that just because some narrow minded people close to them may shut them out for their belief, they won't spend their entire life alone because of it. Cutting people off from outside contact and then instilling the fear of being rejected by everyone you know is one method that cults use to retain members. (Some) Religions use the same basic tactic, just not quite as strict. Atheist websites serve to counter this effort.
To the OP: As others have pointed out, you are already an atheist (assuming that you disbelieve gods). This is not an active choice you make, rather it's a conclusion that you come to based on your experiences/the evidence you've been exposed to. As no one here knows your parents or how they may react, it's hard to give specific advice for you. If you're confident that your parents will be ok with it, just mention to them that you don't believe in their religion anymore, and you'd rather not go to church with them anymore. If you're confident that you're parents will make your life (proverbial) hell for it, then it might be best to go along with them while you live with them, and then tell them once you've moved out and they can't make your life so difficult. If you're in college/university, look around on campus and see if there is a secular/atheist student group where you could meet other people who share your feelings about religion. Feel free to send me a pm if you want some help finding one one your campus/in your city.
On the other hand, not everyone feels strongly enough about it to become such an outspoken atheist, so you may be more comfortable just keeping it to yourself. A great thing about being atheist is that you have no obligations to behave in a certain way. You can be as loud or quiet about it as you want, and there's no magical punishment after you die :D.