How should i come out to my mom as (FTM)?

I know for sure i am. I made a promise to my self that if the packers win the super bowl i would tell her i am . So i want to do it trans gender the problem is i don't know how i should do it i am her only child and i am 13 i don't think she'll take me seriously and she would probally denie things since she always wanted a daughter. so how should i tell her