Take lots of science classes in high school, including biology, chemistry, physics and advanced mathematics. Become as computer literate as possible.
Maintain a high grade point average.
Ask your high school guidance counselor to help you get a list of colleges that are known for their marine biology programs. (Try Marine Biology Web at life.bio.sunysb.edu/marinebio/mbweb.html.)
Send for college catalogs and applications well before your senior year. Make certain you meet the requirements before you apply for admission.
Choose your college carefully, especially if you have a particular emphasis you would like to study. However, realize that your undergraduate studies will be mostly biological science courses that are meant to prepare you for your advanced degree.
Apply for summer internships in your area of interest.
Decide on your eventual specialty in marine biology before you begin to apply to graduate schools for your master's degree. It is pointless to apply to a school where your desired concentration does not currently exist.