Biology classes are a good bet. Math and chemistry will also help if you want to be a vet. You might also see if any local non-profits need volunteers working with animals. They don't have to be marine mammals. You might also want to see if you can learn snorkeling. Local youth groups might have opportunities.

I always suggest that my students find someone who has they job they want and request an informational interview. Just contact the person, tell them that you are a student who is interested in a career in their field and see if you can interview them briefly. First, find out about the job, then think about questions you might ask, like what their most and least favorite parts of the job are. You can also ask their advice on classes or other skills. Have an adult help you look over the questions so that you feel prepared.