Biologists do not normally specialize until they reach graduate level studies. Begin at your local community college and/or university and earn your bachelors degree in biology. Keep your scores high as graduate programs are notoriously competitive to get into.
Marine biologists further specialize into their careers and I’m not sure what you want to study—you didn’t say freshwater or sea water or which particular animal you have an interest in. MBs are needed everywhere from state resource management departments (yes, even the state of Nebraska) to very remote islands on pearl farms and a myriad of roles in between. No matter where you live (yes, even Nebraska) there are aquariums, fisheries centers, state wildlife and conservation offices, and veterinarians who specialize in treating aquatic animals. Find a volunteer position and if any events are held, go and talk to the biologist giving the presentation. You will have a good reference and build your network, but do not let this come before your studies. You will have many people competing for a very limited number of graduate spots.
Be sure during your time as an undergrad, you study for the GRE and submit your application to your graduate school on time, every time. I am partial to the University of Texas A&M at Corpus Christi (have a good hurricane evacuation plan....just saying).