First off, you are joining the Merchant Marine. Not the Merchant Marines. You will be a Mariner not a Marine.
Secondly, before everyone on this website tells you to go and do some searching of previous posts…search previous posts. Lots of good answers there.
It would also help if you gave even the slightest bit of information about yourself. Age, sex, deck or engine, previous experience, are you going to an academy or hawsepiping it, do you have felonies, can you pass a drug test, etc???
Help yourself first by searching and doing research, then if you still can’t find answers to your questions. Help us, help you by giving us as much information to work with as possible. Have you looked at the USCG website to see what’s involved in getting documents for certain endorsements and licenses? The USCG checklists are fantastic for telling you what you need.
Everyone here has had to figure this stuff out just like you will need to. Not trying to sound like a dick, but your question was pretty much the most generic unspecific question ever asked. Haha Not saying we won’t help you figure things out, but you’re giving us nothin to work with.