USMCGrunt is spot on.
In short, you can now reapply for the 4-year National Scholarship during your Freshman year in college. My son was not selected as a High School Senior...went to his college of choice and joined the NROTC unit as a College Programmer. Declared Marine Option and reapplied during his Freshaman year. He was selected but the scholarship didn't kick in until his College Sophomore year. That's pretty much Plan A for those that aren't selected out of High School. Good grades, high PFT score, being active in the unit are very important. The Marine Officer Instructor's recommedation carries alot of weight. If you think about it, you'll have spent several months in daily or weekly contact with the MOI....
If you don't get picked up using that approach, you can stay a College Programmer and apply for what is called a Side Load scholarship...generally those kick in either Sophomore or Junior year. Most apply as Sophomores, get selected and (I think) the actual scholarship kicks in the Junior year. These are rare and getting rarer.
If you've not gotten selected via one of those programs by the end of your Sophomore year, you have to leave the ROTC unit, but there are still other ways:
Separate from ROTC, you can apply for PLC. PLC isn't associated with ROTC, but generally, the Marine PLC candidates do stuff with the Marine Option ROTC students (like PT and Field Exercises).
Last, as mentioned, you can graduate college and apply for OCC.
There are also several other programs for enlisted Marines to either earn a degree + commission (if they don't have a degree) or a commission (if they have a degree).