To my knowledge, there are five paths to commissioning in the U.S. Army.
The classic way is to attend OCS (Officer Candidate School) after first enlisting (I believe you have to wait two years to submit an OCS packet after your enlistment begins, but I may be wrong on the time).
The second, probably most widely known path, is to attend one of the academies (Westpoint for the Army, Annapolis for the Marines and Navy, and the U.S. Air Force Academy for, surprise surprise, the Air Force :lol: ).
There are also several (I believe three, but I may be incorrect on the number) Military Junior Colleges, which are like mini-Academies, and after graduating from one of them (they are two-year schools) you are allowed 36 months to complete your four-year degree, at which time you will receive your commission.
The "Green-to-Gold" program is another path that I am not fully educated on, but from what I have learned from one of my friends who is doing it, an enlisted man leaves the service to attend college, where he earns a four-year degree while working with ROTC, and then re-enters the service upon graduation with his commission.
ROTC (Reserve Officers Training Corps), which is the route I am currently completing), takes college students (civilians who do not necessarily have any prior military service and gives them four years of military training while they are attending school, at the government's expense if they're lucky. After graduation, an ROTC cadet is then commissioned as an O-1 (the most junior officer grade).
Every officer coming out of any of these programs is then sent to an OBC (Officers Basic Course) for their specific MOS (Military Occupational Specialty), after which they are sent to their unit as an officer in the United States military.
Please look any of these programs up on Google if you would like more definite answers, as I said, I am only educated on one of these five approaches, and do not want to give you any misinformation.