The United States Military is comprised of five branches.
These branches are the Army, Navy, Air Force, Marine Corps and Coast Guard. In addition, there is the National Guard.
The Army is primarily responsible for land-based military operations. The Navy, as the name would apply, is responsible for naval-based operations. The Air Force, obviously, is responsible for controlling the airspace of America and it's allies. Lastly, there is the Marine Corps, which was established to be amongst one of the elite fighting forces within the military and fights in all terrains and conditions. The National Guard and Coast Guard are charged with protecting American lands and waters respectively.
When selecting a branch of the military it is important to take into account practical considerations that joining each branch of the military would entail. For instance, if you hate flying, it may be a bad idea to enlist for the Air Force. And if you get sea sick, you probably shouldn't enroll in the Navy or Coast Guard.
The best idea would be to research each branch carefully online here
and then to speak with a recruiter from the branch that you are most interested in.