I think I might be attracted to girls. I haven't done anything with another girl, but I seem to like to look at their bodies and fantasize about them. I'm scared to talk to my parents because they're religious and it's a sin to be gay. What do I do?
The fact that you like to look at other girls' bodies and fantasize about them is a fairly good indication that you're either bisexual or a lesbian, I believe. In fact, you might find it interesting to look at some of the highlights of our survey at http://www.outproud.org/highlights.html, especially on the second page, where there are some charts that illustrate the role that same-gender sexual fantasies have as an indicator as to whether or not you're bisexual or lesbian.
But that's not the real problem from what you write, I believe. The question here, it seems, is whether or not you should tell your parents, and the issues surrounding whether or not it's a sin to be gay.
Here's my thinking on the second question... All evidence seems to suggest that being gay is something you're born with, or as. I have yet to meet anyone who has made a conscious choice to be gay (or heterosexual, for that matter), and is. It's something you're born with and, if your faith would have you believe that God made you as you are, then God made you as a queer youth, and that must just be fine.
There is nothing in the Bible that says that being "gay" is sinful, nor wrong. The concept of "gayness" wasn't even known in Biblical times. The few (there are only six) admonitions against same-sex behavior that are in the Bible have much different contexts, and don't relate to same-gender love as we know and understand it today. (There are literally hundreds of references to opposite-gender sex, but, as one comedian has put it, "perhaps they just need a little more guidance.").
So, from your own standpoint, I hope you know (or come to understand, soon), that whether or not you're gay really shouldn't matter that much to God.
Now, your parents may be another story...
If they are very religious, you need to determine whether or not it's safe for you to tell them. You should be concerned with your own ability to survive, and thrive, first and foremost. If your parents are deeply religious, and their religion is Southern Baptist or something equally as Fundamentalist, the best answer (and it hurts me to say this) is for you to keep your sexual orientation to yourself for awhile longer, and perhaps only tell others in your life with whom you feel safe and can turn to for support. Some ultra-conservative parents have thrown their kids out, others have sent them to reparative therapy camps or psychologists. And none of these are good things.
On the other hand, I have, over the years, seen a wonderful capacity for people to change and grow. I have seen parents who can only describe their earlier behavior as hateful become PFLAG moms and dads, out there carrying banners fighting for their kids' rights. Some of these have even been Fundamentalist and very conservative, so sometimes people will surprise you.
You are in the best position to gauge how your own parents might react. If you think they have the ability to understand this important part of who you are, or the willingness to learn more and find a way to support you out of their love for you, then telling them may be a reality, and ultimately, hopefully a more rewarding journey. But it's important for you to become convinced that this is possible, before embarking on this path. I certainly hope that this is the case, though, and with you all the best.