How do I get my mom to support me being transgender?

If your mom isn't familiar with the topic of transgenders (or LGBTQ+ in general) you can try to educate her by taking her to group support meetings and even have her attends your doctor/therapist etc appointments if allowed. If she is given objective information from professionals it might allow her to be more supportive (especially if you're considering taking hormones/preparing for surgery in the future etc). You can also give her online resources that educate her that is particular to your situation and background, she can even join a support group herself for parents whose children are part of the LGBTQ+ community or call a support hotline. There's no right way of trying to get your mom's support, and you know her better than us, so it depends on what your mom is like and if she's open-minded enough to accept you being transgender. Good luck!