This tool will tell you the frequency, type (axial/tangential/oblique), and locations of room modes based on your room’s dimensions and can be helpful for a rough guide to begin experimenting with room treatment:
How Are You Using Bass Traps?
When it comes to room treatment I found bass traps were easy to deploy. I got dense studio foam bass traps that were 1' x 1' x 2' and placed them right in the front corners roughly from floor to ceiling. Immediate improvement in the tightness and clarity of the bass in my room. What advice would anyone have who wants to use bass traps in their room?