USNA does not offer a Law and Legal studies major. A quick Google search will bring up the majors for USNA. You will not be a JAG (military lawyer) out of USNA. You will be a regular line officer then can apply for the various USMC law programs one you meet the requirements (usually about 2 years into your career you are eligible to start applying). If you google USMC Law Education Programs you can find the two programs available to Active Duty Officers. There are programs to allow Marine Officers to be JAGs immediately when they come in service but they already have a Law degree. This program is extremely competitive and is not an option out of USNA.
There is (or at least was) a Navy Law Education Program, where Navy will send serving officers to Law School in exchange for committment to serve as a JAG after graduation. Like the USMC programs, this is very competitive -- it was only 2 a year back in my day. One of the pilots in my squadron, my predecessor as Squadron Legal Officer, got the program and Navy sent him to University of Chicago Law. I tried to follow a few years later, selected as an alternate, and when I didn't get it..left the Navy to attend law school on my own.
There is (or at least was ) a Law for the Junior Officer course --one of the NL courses back in my day. It was an interesting course, and good info for the junior officer to have. Most units have Legal Officer as a collateral duty, and that is a good job for a JO. You have a lot of interaction with the Skipper, albeit often under less than ideal circumstances.
If you want to be an attorney and attend USNA, the best/easiest way is to go to USNA and the go into an unrestricted line community. If, after a couple of years, you still want to be a lawyer, you can try for one of the programs described above. The program is super-completive and you have to start applying within about 3 yrs of USNA graduation (b/c of how the program is structured). The commitment is (or was) 6 yrs after law school (3 yrs), so you're probably looking at a 20-year career -- and your first tour will almost certainly be on a CVN. OR, get out of the military at some point after your obligation and attend law school on your own. There is PLENTY of time to be a lawyer -- do something interesting first!
If you really want to be a JAG (not as interested in USNA), the better option is to attend whatever undergrad program interests you and then pursue a program whereby the military pays for law school in return for a service commitment. Consult your local officer recruiter for details.
Law for Jr. Officer is still a mandatory course. There are also a number of law courses available through the Political Science dep't -- they are NOT necessary to attend law school or be a lawyer but if you hate them, you'll probably not enjoy law school. Whether you can take them depends on your major.