If you're concerned about what your parents will think enough for that to be one of your main concerns I feel that maybe it is not something that you actually wish to do. Your answers and your posts in other threads do stink rather of trolling. I also don't see why this thread is directed to submissives in particular, anyone can be transgender.
However, as Lestat said, you will need to talk to your doctor, who will then refer you to a psychologist.
The psychologist doesn't just check that you genuinly want it and arn't just trolling the nhs they asses if you have a psychological condition which mimics transgerism (apparantly there are some)
Once they're satisfied you'll be refered on to a gender specialist. They will further asses you and either agree with your diagnosis or not.
Then if they agree with your diagnosis you will be required to undergo councilling, at some point you and your therapist will agree that it is time to begin what is known as social change.
What this means is you will begin to present yourself to the world as the sex you wish to change to. You will need to live, dress, work and everything else as the sex you wish to change to for two years (it is only one year in some other countries but here in the UK it's two). At this point also you will need to change your name.
During this time you will be prescribed hormones of the sex you are changing to.
Once you have completed the two years you will be assessed to see if you are eligible for surgery.
As you can probably see form this it's a long process. Probably an absolute minimum of 3 years, but I think 5 plus is far more common.
Some useful links;
http://www.tsroadmap.com/reality/wpath-standards.html http://www.wpath.org/ http://www.wpath.org/documents2/socv6.pdf