Before you do, you need to make sure it's safe for you to do it. If you think your parents will react badly, you may want to wait until you're on your own so that your basic needs (food, shelter, etc) will still be met. Think about school dynamics, too.
Here's some info to help you think about how you want to handle this.
http://www.plannedparenthood.org/info-for-teens/lgbtq/coming-out-33833.htm
You can also chat with someone about it anonymously at crisischat.org while you decide how you want to move forward.