You are already on the right track by selecting psychology as your undergraduate major. I think you could take two routes after college to pursue your interests. If you are interested in reforming the system and making a difference in policy, you could pursue a Master's degree in Public Policy. If you are interested in helping those that are connected to the criminal justice system and making a difference in lives or the community, you could pursue a Master's degree in Social Work. I suggest looking into each of these graduate degrees and sample courses to see which would align better with your goals.
All the best, Lauren