Just sit down with them and tell them face to face. If that's to much for you try writing it down on piece of paper. Expect them to try to get you to believe in God because as christians that's what many of us are taught to do in those kinds of situations. Don't shut them out, just listen. In the end if they still haven't convinced you respectfully tell them so. If they try telling you that you'll go to hell bring up Matthew Chapter 7 verses 1-5 (Bible New International Version.) 1 “Do not judge, or you too will be judged. 2 For in the same way you judge others, you will be judged, and with the measure you use, it will be measured to you. 3 “Why do you look at the speck of sawdust in your brother’s eye and pay no attention to the plank in your own eye? 4 How can you say to your brother, ‘Let me take the speck out of your eye,’ when all the time there is a plank in your own eye? 5 You hypocrite, first take the plank out of your own eye, and then you will see clearly to remove the speck from your brother’s eye.
The simple answer is, that if you are still underage or living in your parents house, it's probably not a good idea to say anything to them at all, especially if you think they are going to freak out on you. I know that it's frustrating when you don't follow the same beliefs anymore, or if you have found that you aren't sure which path you are following, and you'd like your parents to be supportive of that, but in most cases if there is any indication that they won't be cool with it, it's much better for you to wait until you are older to say anything.
Primarily because your parents have the ability to make your life pretty miserable if they think that you are just doing this to rebel against them, or that you are just doing it out of spite. Secondly because at a younger age they are just so much less likely to take you seriously. So waiting until you are older, gives you more time to settle into your view, and learn as much as possible about them so that you can present a more cohesive argument later on when you do tell them.