The biggest issue with new recruits is RUNNING. Do a lot of that. You do not have to try out for sports or anything organized. You can do that on your own. For The Citadel, there will also be a lot of physical training/conditioning when you enter the program. Even if you are not pursuing ROTC.
With your senior year coming up, you have time to work on your conditioning. Entering any ROTC program will also require some physical training as you go through the program. It is really running (2-3 miles), pushups, situps (or crunches for the Marines), and pull ups. The "advice" is to get off the couch and exercise every day. Do not try to kill yourself at the beginning and get injured. Alternate long runs and shorter, faster runs. Get a good pair of shoes for running. You don't have to go to a gym for pushups and situps, but it can help to do some additional weight training. You could also do some ruck marching and build up distance and weight. That will help get you in shape and strengthen your core muscles.
You don't say where you are from. If you have a recruiter from The Citadel nearby, you can work with them. You can TRY a USMC recruiter, but, keep in mind, they have NOTHING to do with ROTC and they will try to get you to enlist. You are probably better off contacting the NROTC Detachment at The Citadel and asking if they can provide a helpful workout program. This is also a very good excuse to contact them and prove your interest. It could go a LONG way towards successfully receiving a scholarship. Do NOT let your grades slip and do well on the SAT/ACT. Those will be big factors in whether or not you receive a scholarship. Even if you don't get a 4-year, they also have 3 and 2 year scholarships that you can apply for once you are enrolled. You CAN take ROTC without being on scholarship.
Went through Army ROTC, living in South Carolina with friends who are Citadel alumni.