How do I get the Corpsman Rate on the ASVAB?

I'm gonna be taking the ASVAB in a few weeks for the Navy. My dream for awhile, has been to become a Navy Corpsman and to work with the Marines. The only Rate I want is Corpsman and so I'm wondering what I should study for the ASVAB, and what I can do to prepare, and what kind of score do I need to have Corpsman as a choice. I've taken the practice ASVAB at the recruiters and I scored an 80. Any advice would be much appreciated. Thank you.