First I would love to say congratulations on beginning your transition, I think you start when you first accept yourself! First I would like to say that you should prepare yourself for the worst, prepare yourself to lose everyone. I didn't and I am really hurting bad now that I have been losing friends. Find yourself a really nice and fun queer and trans bubble, meet and make new friends, it has been a godsend for me to meet other people and have this amazing rainbow bubble. Lastly I will say it won't be all doom and gloom, you will most likely not lose everyone and may even become closer to a lot of people.
As with your girlfriend you should be honest with her that you need to transition at your own pace, ultimately this is your decision and no one elses. Your therapist and doctors are there to help you throughout. Honestly you never know how your relationship with her will be during and after transition. I've heard every story under the sun from horrible break ups, to becoming close friends rather than loves to spats ending with keeping and strengthening relationships. You definitely need to sit down and really talk to her and everyone else important about this. For me it helped getting some people to understand my transition.
Also you are never too young to transition and be yourself, some are lucky to start before puberty and others will start when they are elderly. Do what makes you happy and remember your mileage will vary, some will pass better than others tbh. Just remember you will never know until you try. Also starting gradually by changing various aspects of your wardrobe will help yourself and others adjust plus you'll find there are just some clothes that won't fit until you get your figure. Anyways best of luck with everything and welcome to Susans :]
