If you are in the military you need to contact your Unit Marksmanship Coordinator. If you are National Guard your unit may not have a designated UMC, so you can try to contact the State Marksmanship Coordinator. You then need to join up with your Battalion or Brigade level marksmanship team, and then compete to earn a spot on your Division, State, or Regional team. Then you can start working towards the President's Hundred and/or Governor's Twenty.
If you are civilian, I have no idea.