I would recommend that you become really good at public speaking, perhaps join the local Toastmaster's/Toastmistress organization if they accept teenagers. Get involved in debate.
Above all else, truly learn grammar. I have seen too many cases that came down to a discussion on the structure of the sentence and parts of speech, and they made absolutely no sense to me!
As we are moving (hopefully) more towards alternative sentencing, you will want to understand some of the social problems we, as a society, are facing. Drugs and alcohol, veteran's problems, recidivism, re-employment problems faced by ex-cons, etc. The jails and prisons are over-crowded, people who go there meet other "bad" people and sometimes turn more towards crime than they would have had we kept them out of prison. Sending someone to prison is hard on families, may make them have to turn to welfare, etc.
I am a retired police officer. Yes, we still need to lock up some people. But, sometimes, there are alternatives. I think we are starting to realize this, and think as a prosecutor you should have an understanding of the whole process, and not just the prosecutor's role.