A great first step is to start volunteering at a local veterinarian office. This is a good way to see what the job is really like and also get some real world experience while making contacts that could later become professional references.
You should also focus on math and science courses as you will major in these in college. If you can take college prep or AP courses, this will also help prepare you well for college. As you think about college, you will want to look for schools with strong science programs.