I hate to say it, but I wouldn't start off with the word "atheist". Losing your faith and not being religious is one thing, but atheism currently has a special stigma among Christian zealots of all kinds.
If it's important enough to you to be fully honest with people, I would simply say to selected people that you don't believe in God (anymore, if you ever did). Don't deny atheism if you're accused of it, but don't bring it up yourself.
Once it's out there, you'll have to explain yourself. Not just yourself, but all atheists - you'll find all kinds of misconceptions about them: that they hate God, that they worship Satan, that they want to force people to give up religion or remove kids from religious parents and so on.
Besides that you'll get all the really obvious, kneejerk arguments for God, like the beauty of a flower and "where did it all come from". Your family may send you to a preacher or even a Bible camp if they don't trust their own ability to proselytise. This site and the rest of the Web provides answers to pretty much all of it. Bone up. Start by searching for my "Great Big Arguments" series.
Once you've established that you've thought this through and you can't be reconverted with simple platitudes, if your community is as religious as you say it is then you will lose friends. Sorry, but some people don't like to be around atheists, who question their lifelong assumptions by just existing.
You may also find that you become better friends with others; those who are also quietly questioning their faith. When Richard Dawkins did a lecture tour through the Bible Belt, he found big, enthusiastic audiences everyplace. The irreligious are everywhere, as paranoid Christians often warn. Hopefully you'll find some more of them.
Let us know how you go, if you go ahead with it. Good luck.