Which walls are the front and back. 14ft or 24ft ?
Do you have windows on any of the walls ?
Floor to ceiling bass traps as others suggest , then in my opinion buy 2 @ 2X4 2" thick absorbers and 2 of the 2D or curved diffusers , then place them in the front middle and side walls , then swap them . this is the only way to determine what sounds best to you . Then you can add more as needed or wanted .