I'm assuming you're in a country like the US where, legally, being older than 18 means you're legally an adult.
My advice? Get the ball rolling on transition, if its what you want to do. It's your life, and frankly nobody can stop you. You might owe your mom some time to come to terms with who you are, and there's plenty you can do to attempt to help her come around, but at the end of the day you don't owe her any grace period around transition.
If you think it's right for you, I'd start looking for a therapist and a doctor/endocrinologist, and try to get the balls (lol balls) rolling. If, again, and of course, you feel like this is what's right for you.
Congratulations on coming out!