Make it clear to your daughter that it's entirely her choice how she wants to live her life once she is grown up. Tell her you will stand by her regardless of how she lives it. Stress that you stand by her even in this difficult times.

Maybe she wants to be called "he" from now on, I wouldn't deny this. If you do she/he will probably distance himself/herself from you. Maybe he/she wants to have some boy clothes, buy them. These are easy things which don't harm anybody.

You should definitely get help from a medical professional. I would suggest talking to more than one psychologist. Also get one for yourself. You will not be able to cope with this situation on your own.

The decision to use drugs should not be taken lightly. The drugs in question, so called gonadotropin-releasing hormone (GnRH) analogues, have adverse effects on bone mineralisation and compromise fertility; data on the effects on brain development are still limited [\[1\]][1]. The desired (not the adverse) effects of the drug are reversible though [\[1\]][1]. 

It's important to find unbiased medical help. In some countries like the US the debate is extremely politically polarized on both sides of the spectrum. You want to stay away from extreme conservatives and also extreme gender progressives. As your child is still very young both groups can heavily influence her. Find neutral advise.

This is why it's important to realize that she can't make the choice to get surgery on her own, no matter how bad she/her wants it. She is still very young and her views on many topics not yet fully established and can easily be manipulated. This is not limited to the topic of gender and that's why you decide about her, not she.

Advise her/him to first try her new lifestyle at home in a safe space for some time. You should make it very clear to her what the possible consequences may be in the outside world, while stressing that you stand with her/him regardless. 

Prevent her/him from rushing things. This process will take years and children change. You want to prevent her from doing something she later regrets and stretch the process out over many years in order to do so. Be clear about that: Tell her this is not going to happen in a matter of days, weeks or months. 

**The most important thing is that you need to have long talks about identity. Soley changing gender will probably not make her happy. I for myself often thought that getting a top grade in university, a certain well-paid job or a new partner will make me happy, but this has been naive. Happiness is nothing you acquire by some surgery or some single event.**


  [1]: https://www.sciencedirect.com/science/article/pii/S2444866417301101