Go to the website of each ROTC program you would like to apply to and research, taking notes.
Before you can submit your ROTC application(s), you will need to decide what you want to major in and which are your top five colleges. This can be a tough decision, so I recommend you start working on that now. Some ROTC programs have tiered majors, while others don’t. Research which colleges have good ROTC programs and also which would be good for your intended major, and potentially which colleges could be affordable in the event that the ROTC scholarship doesn’t work out for some reason while you are attending college. This list is takes time to put together.
Research what the physical fitness test requires, and make a plan to train for it. If you are applying to more than one ROTC and/or one or more academies, they may all require different physical fitness tests with different length runs, etc. For example, AROTC requires a one-mile run, and NROTC-MO requires a 3-mile run. You would need to train separately for these tests. Make goals for when you want to complete the tests and a time-frame for training for them. This can be tricky if you do varsity sports and are busy with that. Summer is often a convenient time to get this component done.
Another thing to do is research which teacher recommendations you will need for each application. Some teachers go off the grid during summer and could be impossible to reach, say in July or August when you really need them to complete your recommendations. If you get their contact information before the school year ends in June, it will be a lot easier to contact them this summer if you need to.
Finally, research what the dates are for each ROTC board. They usually have two or three boards during the year. If you get your application done early for the first board in the fall, it could potentially be strategic, if you have a strong application. However, if your application will not be strongest in the fall (say you need to improve your SAT score), then you may want to submit it later so that you have the best chance possible to get a scholarship, even though you may be competing on fewer boards. Your application will only be reviewed once by one board. Try to submit it as early as you can but also make it as strong as you can.
My son got AROTC and NROTC-MO scholarships this year. He also applied to USNA and USMA. That was a huge project to finish, so he needed to start early. If you have specific questions about those, I can try to help.