Jem:
First off, the masulinizing effects of testosterone actually last well into the twenties, so stopping now will have more of an effect than you think, really.
Also the feminizing effects of estrogen can be very strong when you are younger and actually reverse some of the soft tissue development. You really are not too late. Take it from an *old* woman of 55! I started HRT just about 18 months ago and it still had quite an effect on me! You'll be fine.
I don't know what part of the world you are from, but if you are in the United States, the standard process of going on HRT is to see a therapist for at least three months, then that therapist would write a letter for you to an endocrinologist indicating that the therapist feels that you are a transsexual and a good candidate for hormone replacement therapy. The endo will run a blood test on you to make sure that you are healthy enough for HRT and then start you on a regimen of lower doses of anti-androgens and estrogen. Then after about three months run the blood test again and see how your body is responding to the feminizing effects of HRT. As time goes along the endo may recommend higher levels of estrogen. In a really short period of time after starting HRT, you will start to notice feminizing effects such as body hair start to thin and some breast tenderness and swelling. As time goes along these effects will become more pronounced and such things as your libido will be effected, you will lose muscle mass, your testicles will start to shrink and spontaneous erections will happen less often.
Among the things the endo will check for in the tests are liver function, testosterone and estrogen levels.
Please Jem, don't give up hope. It sounds like you are actually in a pretty good condition. If you have come out to your parents and your sister already knows, than you really have gotten over the hard part! Acceptance can be the most difficult thing to achieve so if you are starting out with that you will find that going forward will be easier to do.
And you have all your friends here to support you, so anytime you need help, just call!
-Sandy