Some good advice above about getting into shape. A couple of additional tips. They may be more applicable to AROTC cadets not at the SMCs where student life is under stricter control.
1. Leave the X-Box at Home
2. Do Not Get ****-Faced Drunk
Time Management - Leave the X-Box Home
You will enjoy unprecedented freedom at college. In high school you showed up in the morning, spent the day in classes, then practiced afterwards for varsity sports. Most college classes don't meet every day, and generally nobody cares if you cut class (except ROTC classes). For that matter, nobody cares if you eat nutritious meals or fuel yourself on cokes, cookies and doritos, and nobody cares if you get sufficient sleep or stay up until 4 a.m. to master Dragon Ball Xenoverse.
You will be competing with fellow cadets in your battalion for slots at training schools such as Airborne and Air Assault. If you set a goal of achieving Active Duty status, you will be competing with similarly-minded cadets across the country. You may not have this goal when you arrive, and very likely you will not have a specific branch in mind. Never mind that - you will later on. Put yourself in a position that your 21 year-old-self is grateful to the 18 year-old you are now. Build a strong GPA and score high in fitness tests, and you will be well on your way toward that goal. However, this requires that you use your time efficiently. Have fun in college, party, join a frat, but make sure you take care of business first.
Alcohol Management - Do Not Get ****-Faced Drunk
Your new freedoms will also include the ability to obtain alcohol. In most colleges alcohol is easy to get whether or not you are of legal age. Frats are always having parties, many bars and liquor stores rely on student business, and if they do card you there is always an older dorm-mate. But the lure of alcohol has wrecked the paths of many ROTC cadets and damaged the careers of many officers. There are great stories of the wild and stupid things cadets have gotten away with, but it often ends badly. I'm not so naive to think ROTC cadets won't drink, even if they're not supposed to. But you need to know how to restrain yourself. Every party does not require your attendance and every keg does not require you to sample it.