You don't mention it in your question, but what is your experience? You say you want to work at an awesome startup, but do you have projects (such as websites) or anything you can reference to prove you know what you're doing?

San Francisco is a great place, but you're already going to be fighting against quite a lot of people with more experience. Unless you some impressive projects to show, finding a startup who is going to take a risk on a 19 year is not a good bet to make.