Talk to the coach. Get a training plan for the summer.
If your school has a fall cross country program, consider joining it, and working out with the runners over the summer. If your school has a winter track program, join that, too.
Whether you make the varsity team or not may depend on local circumstances. If you are in a large school, in which several girls join the track team, you will have more competition to make varsity. If you are at a small school, you have a good chance of making the varsity team.