Well, I was going to save this til the morning, but since I can't go to sleep, here's my advice!
The most important thing to remember is: what do you want to accomplish? Are you just trying to gain acceptance or do you have bigger fish to fry? This is important to remember what your goal is because it is super easy to get derailed by questions and attempting to explain yourself. I have a saying I like, "we should always seek to understand, but occasionally, acceptance comes before understanding." This may be what you are trying to do: get them to accept that your ideas are changing with out giving them, too much reason for why. Maybe you are trying to get them to understand you, in which case you may need to educate yourself (and them) on many different ideas from scientific concepts such as evolution and cosmology to basic philosophical arguments and modes of ethics to human/ancient/church history and possibly all matter of subjects depending on the background of your parents. What can I say except that there's a lot of good reasons not to believe?
I suggest not telling them together. Tell which ever parent you feel is most receptive first. They may team up on you if they are together, but separately, it's a one on one conversation and that parent may give you some insight on how best to approach the other. Sometimes, it's best to first tell a friend or sibling who you know will be supportive of you so you physically have someone in your life who you can talk to about how your parents are taking the news. Here's a link with some good tips. You are still in High School and you are still young, that means you might get the "you are just in a phase" response. If the conversation is going badly, you can use that for an out if it comes up. You can also use your age to your benefit. If you are getting a lot of push back you can resort to the line similar to... "I can tell this is upsetting you and I realize that I still have a lot to learn, but I'm trying to figure it all out, So please have patience with me, and all I ask is that you don't push me into any particular direction. This is something I feel I need to determine for myself."
You don't have to tell them you are an atheist, especially if you haven't yet decided that you are. About a year and a half before I told my parents I no longer believed I first told my father I was pretty much an agnostic. He is a very Catholic man and was constantly asking me how church was and if I had found a good one to attend regularly. I probably lied to him for the better part of a year, which I still regret that I did and that it felt necessary to do, in order to keep him from finding out even despite the fact that I had been on my own for almost five years by that point. I simply told him that at that point in my life I wasn't sure, and that what we thought we knew about a god, I couldn't be sure was what that god was like.
He accepted it at the time and encouraged me not to quit without essentially giving it a good try. Later when I told him I was an atheist, He told me that he also went through a period of doubt at the age I was, so looking back I can see why he was so understanding at the time.
As we caution all our young, non-believers who must still live at home, if you feel telling them this will cause significant hardship for you, then there is no shame in not telling them until you are out of their house and liberated from their support. Only you can make that determination as you know your parents better than us. They might surprise you, which can really go either way.
And if this hasn't been helpful just do a Google search for "how to tell my parents I'm agnostic/atheist" and you get lots of different discussions from all around the internet. I saw one from the Minecraft forums!
I went through a fairly similar thing with my parents.
I started by expressing my doubts and questions about the bible to my parents, and then (partly on their advise) also to the pastor.
The pastor didn't help too much. We fairly soon realized (and he accepted) that the usual circular reason doesn't work if you don't already (or anymore) believe. So he said he would pray for me, and we left it at that.
My parents were very understanding, and I found out my father has a similar view on religion and the bible as I have. To him, faith is a personal (and meditative) affair and the bible is a guideline written by humans (who may or may not be inspired, or had their own agenda). They quickly accepted my agnosticism and later atheism , and it has never been a real issue.
To my family, church has alway been more about the social side than the 'true' religious side. And when I visit my parents on a Sunday, I tend to go with them to church (mainly to reacquaint with old friends).
"Pass the salt. There is no God. What's for dessert?"
It's simple, really - arrange for a sleepover at a friend's house, then call them at 3 in the morning and tell them you've been in a horrific accident, but doctors believe that with years of intensive therapy, you may regain partial use of the left side of your body - then just say, "Not really, I've just decided I'm an atheist --"
Of course, this only works once.
But seriously, Sagacious Hawk, as usual, has suggested an excellent approach, especially the part of telling them one at a time. But again, as some have said, I wouldn't just leap into it - as you, yourself have made clear, you aren't really sure yet what your own position is. Do this over a period of time, rather than Unseen's UN-sagacious, "Pass the salt. There is no God. What's for dessert?" possibly by asking questions of each of them, that will require them to think before they answer.
"Why would god confuse the language and scatter the people who were trying to build a tower to heaven, if he knew they'd just pass out from lack of oxygen if they got high enough?"
"How could Noah's flood cover all the mountains and 15 cubits above that, when the National Geological Survey says there isn't enough water in, on, above, or under the earth to reach high enough to cover even more than half of Mt. Ararat, much less Mt. Everest?"
"Where did Cain's wife come from?"
"When Sodom and Gomorrah were destroyed, do you think it helped that the two cities sat on a major fault line that ran all the way from the Olduvai Gorge in Africa, and were prone to massive earthquakes?"
Trust me, there are a million other questions you could ask as well, but you would need to actually do your own research, in order to find them, which will help you in your own quest for your philosophical identity. And it will let them see, little by little, rather than in one sledgehammer blow, that you're beginning to think for yourself.
Of course, if they're typical, they'll try to pass the buck by suggesting you ask your minister, but that's when you say, "No, I want to know what YOU think --" it could well be that no one has ever asked them that before, and it will certainly give you an insight into where they're coming from, which leaves you better prepared for the next round.