I don't know how much research you already did, but it would be good to read about the issue and more importanrly to talk live to people who went through the same thing as you. Look for a transgendered organisation or support group in your vicinity. If you don't know where to look start whith gay/lesbian support assocations, they may be able to direct you. Anyway your aim would be to better understand from people with real life experience what you want, how to achieve it, and avoid repeating mistakes others did.

As for your girlfriend, it's a difficult situation. There's no guarantee she will wish to live with the person you want to becme, but OTOH the issue will inevitably have to be dealt with. I think the best you can do is to facilitate things to her, by explaining and answering her questions, and showing that you still care for her. She will be very anxious, so it will be important to answer in a clear way to her questions, hence the utility of speaking with people who know. In the ideal situation she would actually support you, but you have to accept that it's not certain and make your best to help her keep up with you.

Good luck, and if possible do let us know how you are progressing.