Coming out is the most difficult thing I did with my family. We have been through so many things together (and most of it was my own stupidity), but coming out was the hardest.
I got lucky, my family accepted it. Not everyone is as fortunate. The last two posts give the best advice.
Sit down with your parents and talk to them. Talk to your siblings, as well. If you have a sibling on your side and your mom on your side, you have got the entire house on your side. Like has been previously mentioned, if you are concerned about being kicked out of the house, do not say anything until you have established yourself elsewhere. If you think your mom can accept your homosexuality, talk to her first and have her sit with you and your dad. You may also want to consider offering your mother some literature from PFLAG. They have some great literature which will help you and your family come to terms with your homosexuality.
Coming out is a very personal thing and a very personal challenge. It is also one of the most liberating things that can happen to you.
Please, be safe and think very hard about what you are going to say to your parents. Remember, after you have come to terms with who you are, they have to come to terms with who you are. They have their own "coming out process" that they have to go through. Give them time and give them space. They are going to need it. And no matter what happens, do not fight with them and constantly re-assure them that you love them, no matter what.
Good Luck
PJ