Ok, as someone who's been in this situation before, I have some advice.
First off, never ever tell them you became atheist because you don't care. One, that's not true at all, because if you REALLY TRULY didn't care, you wouldn't be here right now. You would have just kept on going to church, and believing in god because you would have never sat down and actually thought about it, and whether or not you believe in it. Second, it kind of comes off as rude, because obviously if you think this will be an issue THEY CARE VERY DEEPLY. How would you like it if say, you told someone that you loved them and they said they didn't care? It would kind of sting, wouldn't it?
Another thing is to make sure they understand that this doesn't change anything else. You're still the same person. You're not going to suddenly up and start killing people because "there's no god so it doesn't matter" or whatever it is they think atheists do.
Do go to church with them sometimes. This proves to them that you understand that even if it doesn't mean anything to you, it does mean something to them, and that you are still willing to at least TRY to understand their point of view. Don't make snide remarks, be respectful of other's beliefs. This makes others more willing to accept yours.
Don't shut them out whenever they start going on about faith. It helps greatly if you can keep an open forum of discussion, without it devolving into a fight.
Be informed. Don't just say things like "Well I'm an atheist because god never answered my prayers for a pony!" Do research on biblical history. It makes you sound more rational if you say "The bible in its current form wasn't written down until a few hundred years ago, the earliest forms weren't written down for a few thousand years before that, and before even that its was a series of oral traditions from the middle east, so its text has always been in a constant state of flux" than if you say "THE BIBLE IS WRONG BLUH BLUH IM AN ATHEIST!"
Small doses help. Try have one big spiel and then drop the subject entirely. If this is going to be a shock to them its going to take them a long time to process. The first time you tell them is going to be the first time of MANY.
Understand that they might NEVER understand. My parents got over it, and are very accepting now. My grandmother still cries when she hears I'm still not going to church. I first told her when I was 12. I'm 23 now.
That's pretty much all the advice I can give you. The most important thing is to not be antagonistic about things. There's a difference between being an atheist and being an arrogant jerk.