Your own plebe year experience and performance will be a good indicator. You'll be taking typical plebe courses and getting briefs on majors, while observing and talking to upperclass in your company, sports team and ECAs about all kinds of things, including majors, service selection, summer training, etc.
Declaring a major happens later on. See how you do with managing USNA academics and courses with labs, your sports team, plebe pro quals, military duties and overall time management. Be open to all the majors. You will have plenty of time to make a decision.
Much will depend on you. Your classmates will have similar HS performance backgrounds in terms of abilities, grades and test scores. It is a fact of life some in your class will now get the C, D and F grades. Balancing everything is something you will learn, sometimes the hard way. You will learn to optimize performance over a range of competing demands, rather than maximizing. The key is to be sat in all areas, academic, athletic/physical, military performance, conduct. You will only have x hours in your day, and quite often you don't get much say in how you spend them. You will learn how to invest them and become very focused and efficient. When the going gets tough, you might settle for a B in a 3-credit course because you are spending extra time on a 5-credit course to bring your C up to a B.
In the meantime, finish strong what you're doing now, enjoy this time at the top of the school heap, fine-tune your fitness (many threads on this) and score "good kid" points with your family.