How should I go about asking my mom for birth control?

The best thing to do, is just come out and say it. Ask your mom to go for a drive, where you can be alone and talk in private. Tell her the facts, you are SEVENTEEN, going to college, and are not inexperienced. Tell her you've been safe, but want to be 100% sure all the time. Since she is a woman, she should understand. It sounds to me like you are a pretty responsible person, so be an adult and tell her.