Its rough man. My Mother is a very staunch Christian and took the news about how I expected. Crying. I understood her, she felt her son was going to go to hell. I am not going to lie to you and say everything will be okay, because just stating facts to someone who believes something on faith does very very little.
My advice is be patient, be kind, but be firm. Make sure that they know this is not a decision you have made lightly, that you have considered it for sometime, weighed the evidence for God and found it lacking.
I cannot tell from your post how long ago you made this decision, if it was literally in the last couple days I would advise waiting a few days before talking to your parents etc. I personally just deconverted about 7 months ago and I still feel like I learn new things everyday... granted I spend a minimum of an hour a day on atheist sites and on YouTube making sure I understand my new (lack of) faith. If it is something new for you I would youtube search Christopher Hitchens, Sam Harris, or Richard Dawkins and listen to their arguments (The Atheist Experience is also a pretty good show out of Austin TX, but available on YouTube). I also strongly recommend A Letter to a Christian Nation by Sam Harris; its short (not so) sweet and too the point (and you can download a pdf file for free).