So you're saying you picked natures and coverage moves randomly? My last comment should have been pretty clear about how you can pick better natures. As for coverage moves, find moves that are reasonably powerful and match your Pokemon's category preference (physical attacks for physical Pokemon, special attacks for special Pokemon). After listing some of those moves, use the move(s) that are strong against the most types that resist your other moves. If you get a tie, you should probably just use the move with more base power.
In case you don't know already, using a monotype team will cause your Pokemon to share a lot of weaknesses. That being said, good monotype teams have Pokemon that cover the team's shared weaknesses as much as possible. Pokemon with one type will serve to worsen those weaknesses. Good types for a psychic team are fighting (not weak to dark and bug, hits dark and steel strongly), dark (not weak to ghost and dark, hits psychic and ghost strongly), and fire (not weak to bug, hits bug and steel strongly). After you have those three types, try to get as much type variety and coverage as you can fit on one team.