A number of replies said that your parents may already sense that you're gay. Speaking of my own experience, I was amazed how much parents will block out, if they want to (I was a student at the time and they had visited me in my student flat, where I had a wall full of black&white postcards of half naked men. Yet they did not bat an eye lid!)
Anyway, I understand why you want to tell them. Sexuality is a major factor in anybody's life and whilst your parents don't need to know what you get up to in the bedroom, you want them to know who you love (big difference, by the way).
There really is no right or wrong way to go about it. If you have a sibling you can trust, speak to them first. Or if you have a boyf or gay friends, let them know when you're about to come out, so they can be there for you to support you.
Obviously, try to catch your parents at a quiet time. Also, remember how long you may have gone over this again and again, so don't expect your parents to be fine with this within a few days - they are only human.
My parents were both 60 when I told them and I got the classic reaction: mum burst into tears, dad sat there stoney-faced. To their credit, the first thing they said was that I was and will be their son, no matter what. Even though it has been a long and sometimes slow treck, they did come round eventually.
Just make sure that you also remain open to answer any questions they may have or, if your parents would like it, put them in touch with a parent support group. You can contact London Lesbian&Gay Switchboard for any further tips and info.
Good luck!