1. Go to the one or two Universities or Colleges you are most interested in at this time, and go to the school's website. Then go to the Department you are interested in: e.g. go to Google, put in: "Baylor University" "Department of Biology", and you will be directed to the Biology Dept. at Baylor (or whatever college). There if you hunt around, it will tell you what the major is about, what classes you have to take (for example, at Baylor a Bio major must take 16 hours of Chemistry, 8 hours of Calculus, 8 hours of Calculus based Physics, and 4 hours of Geology, along with about 32 hours of Biology requisites and Electives.
Do the same for your second choice of major, and your third choice.
2. In general, the Military needs engineers who are physically fit and strong leaders. The Air Force and Navy both are HIGHLY Engineeering focused, with a second focus on Math, Science. Navy ROTC allocates 85% of scholarship dollars to Engineering, Science and Math majors. The Army is starting to become serious about Science, Technology, Engineering and Math (aka STEM). I was told next year, that is THIS application class of FY 2016, an award winner will not be able to change majors from Engineering to Non-Engineering, or Science/Math to Non-Science/Math, without Battalion (or possibly Bridade or Cadet Command) approval. So now even Army is going to spend its scholarship dollars for STEM students mostly. However, most Army ROTC majors are currently not STEM.
3. All of the ROTC programs, when evaluating you for positions of leadership and further training, put the most emphasis on GPA. Since most students get higher grades in subjects they're actually interested in, I would suggest picking both a major, and a college, where you will feel motivated, engaged, and excited. To attempt to pick a major you're not interested in just to increase the chances of winning a scholarship is really putting the cart before the horse, and will 90% of the time end in pain and frustration. Doing poorly in a major not well suited to you will hurt your GPA (and possibly get you kicked out of that major), which will hurt your evaluation within ROTC. Currentlly only 60% of the Army ROTC grads go into Active Duty, with 40% commissioning into Reserves of National Guard -- and most of these wanted Active Duty, but their postion on the Order of Merit List, which again is mostly about GPA, was too low. In five years, it is likely, based on what we can observe now, for perhaps only 40% of Army ROTC grads to get to go Active Duty. Again, GPA will be the most important determinant in this. Bottom line -- pick a major you're actually interested in, and show some competence in, so that your grades will be high.
A 4th question which is implied by our post is: What College should I pick? The ROTC PMSs I've spoken with all agree... pick the college where you will feel the most comfortable, the happiest. Again, that's where you're likely to do the best in the classroom = higher GPA = more opportunities withing ROTC and upon commissioning.