Believe it or not, plenty of families are totally supportive of their gay teens. Of course, many teens know that coming out at home just would not be safe or wise.
If that is the case, you might still have another relative who you can confide in. If mom and dad aren't options, consider talking to a sibling, aunt, uncle or grandparent.
It is also important to remember that parents may not react exactly how you'd like when you come out. But try to remember, in most cases, you have had a lot more time to process this news about your identity than they have. They might ask what seem like ignorant questions ("How can you know for sure you're a lesbian?") or make annoying comments ("Guys can't be bisexual, you know."), but often this is just their way of sorting through something they know very little about. Even if they don't seem supportive at first, a lot of parents will eventually come around over time!
It can be really helpful to have a list of good, accurate resources from places your parents are likely to trust--say, a statement by the American Psychological Association saying that sexual orientation cannot be changed, and trying to do so can be harmful.You might also want to gather a list of websites and identify some adult allies who you think your parents would listen to. Sometimes talking to your family about sexual orientation can be really intense and emotional, so it can be helpful to be able to say, "You know what, maybe Aunt Tillie can explain this better," and then have her do so.
Lastly, you need to know about Parents and Friends of Lesbians and Gays, a great organization, which offers information and help for for families and individuals, and which can also give you some tips on helping your parents support you.