You don't say what branch ROTC. If you are speaking of Army ROTC, I can tell you that my son joined Army ROTC without a contract or scholarship at a large public university with a medium sized Battalion as a freshman and was successful at earning a 3 year campus based AROTC scholarship by his third month in the program. The following January, they added another semester making it a total of 3 1/2 years.
Was it easy? Not exactly. He did keep a B+ average while carrying 20 semester units, plus he was one of the top cadets in the battalion on two of the three APFT metrics (lousy runner) and volunteered for lots of EC's. His cadre looked at effort, attitude, grades, SAT/ACT, and APFT. It may have helped that he had already passed DoDMERB when he originally applied to the SA's.
The bottom line though is that if you are seeking to join ROTC just for the money, it is NOT a good idea. The work can be a high stress grind on top of a lot of time demands. This combined with the service commitment will challenge the majority of students who seek strictly a financial benefit. My son went in to the program thinking that it would be relatively easy. There were times on LDX when he was lugging a full ruck plus an M-240 with ammo uphill that he was driven close to tears. He nonetheless pushed through it and he is glad that he did.
People who truly thrive
in ROTC are those who are determined to serve their country first and the money is just a bonus.
It doesn't hurt to enroll in the program, and get a feel for it. If you are enrolling as a sophomore you will likely need to attend CIET in the summer to get caught up on what you missed in your freshman year. AROTC offers campus based 2, and 3 year scholarships. The first two years is called the Basic course. Anyone can enroll in the Basic course. the Junior and Senior years is considered the Advanced course which you must qualify for. Also there is no military obligation until you are offered a scholarship or contract.
Here is an excellent link to one campus based AROTC website from UC Davis. It answers a lot of FAQ's about enrolling without a contract or scholarship: