My mom gave me her Last Name/ on my birth certificate. How can/what do I have to do to change my name to my father's last name? Legally!
You can do so when you turn 18
Get a lawyer and he will file the necessary paperwork.
Pending on the state you can ask your magistrate it costs 40 bucks or more but if your under 18 both parents have to sign if your over 18 just pay and change it