I don't know that it will impact your application much. But it may give you a little taste of the military. Its a pretty distant taste, but its a little.
If you want to do it because you want to learn a little more about the military and help you make a decision, then by all means do it. If you think having JROTC on your application is going to propel you forward in the process, then you will likely be disappointed.
Get great grades, score high on SAT/ACT, play a team sport and demonstrate leadership through a club, team, student government or other activity.
Extras like Boys State, JROTC, CAP, etc are not the meat of the application. They can help, but they do not carry the day. What makes them help more is if you actually do something meaningful with those experiences. If you can write about how they helped you a certain way or how you grew from the experience or learned your leadership style then great. Just having them on the application itself is better than not having them, but don't ever think that they will make up for poor grades, and/or low test score. I see a lot of people over emphasize these extras instead of doubling their efforts on grades and test scores to their detriment.
