A .20 ga is fine for a turkey gun, but you'll want to go to a tighter choke than you would for bird hunting. Pattern the gun to find it's effective range and don't shoot at anything outside of that. If I were trying to learn 1 call I'd go with a slate/peg and practice, practice, practice. Don't call too much, if a bird is responding to your calls, shut up and get ready to shoot. Most importantly, as someone else said, know your prey. Read as much as you can about turkeys and turkey hunting and have fun! Good luck.