Each school has different pre-requisite requirements - which makes it hard to know what to take. You almost have to know what school you're going to, to know what pre-reqs you need. And even then, you may not get in - many nursing schools
now have long waiting lists, and are very competitive to get in. Make sure you get the highest grades possible to increase your chances.
Anyway, I don't know about the pre-reqs for an ADN - I only know the pre-reqs for a BSN - these include classes like Biology, Microbiology, Anatomy, Physiology, Psychology, Sociology, Lifespan Development, and sometimes Chemisty, Nutrition, Statistics, and Algebra. Again, each school is different and wants different classes. I would start looking at schools online and read what their requirements are. Maybe even meet with an academic counselor to help you map out your classes.
Becoming a medical assistant is nice to be introduced into a clinical setting, but it doesn't help you with getting into nursing school
or becoming a nurse. To become a nurse you have to graduate from an accredited program and then pass the NCLEX exam (state board).
One option might be to become an LPN (usually a year in school), start working, and then take the remaining classes to get your ADN or BSN. With your ADN or BSN, then you can take the NCLEX exam and become an RN.
By the way - what's a Hospital Corpsman?