For freshman, the way to start is by enrolling in MILS 1011 or MILS 1012 'First Year Basic Army.' This is the basic ROTC class; it does not require any military obligation from those students who choose to enroll.
The great part about this class is that you get to see the many incredible opportunities the Army ROTC has in store for you. In addition, ROTC classes count as electives in most major degree programs.
At the end of the year, if you are still interested in becoming a contracted member of the Army ROTC program, you can continue to take Basic ROTC courses until the end of your sophomore year. (Still, with no military obligation!).
If you think Army ROTC and the prestige of an Army Officer's Commission is for you, you would then follow a sequence of courses that will take you through your junior and senior years in the Advanced Army ROTC program. As a junior you will be required to contract with the Army and obligate to at least 4 years of service.
Upon completion, your course work will result in a commission in the United States Army (Active Duty, National Guard or Army Reserve).