How do I get birth controll?

My little sister who is 15 told me today that she has had sex. And plans to on "special occasions" .. but we know that could be any night. I told her she NEEDS to be on birth control and I would help her get it but I have nO IDEA HOW! I am 20 but I have never had sex because I am waiting till I'm married so I don't even know where to go from here. Do I have to go to a doctor? I can't tell anyone because I am the only person she trusts. I don't know what to do...