If you feel comfortable talking with them about things, you may just want to sit them down and do a "i've been doing some research and talked to my doctor, i believe i am transgender/transsexual and i will be going to therapy to figure it out."
Offer print out info or books if they take it well enough (have them ready). Be calm, collected, and rational. Try not to panic (i tend to panic when it comes to personal, potentially BIG news.) Be mature about it.
Try to stress it isn't a CHOICE, it is just there. Have some links to research, maybe. You don't WANT to be a boy, you FEEL like a boy and wish to have your outside match your inside (so to speak.)
If you feel more comfortable talking to one over the other, go to that one first. I don't really feel comfortable talking to either of my parents (they aren't very open, though i did have an interesting conversation with my mom once about my sexual orientation...) so i went the write-a-letter-and-have-them-read-it route.
I was all set to leave them the letter yesterday (i'm very proud of my letter, BTW, spent hours on it as well as compiling a bit of info for them lol - if you want it as a basis, i'm willing to share just PM me) even though we're all sick. Things are just too far along, a lot in motion and i don't feel right hiding it too much longer.
But then my dad had to go to the doc and turned out he has pneumonia.

I'm debating just giving my MOM the letter and letting HER show my dad when she thinks he can handle it... or just waiting.