Room treatment refers to the modification of the room to improve sound quality.
1. Reflections - The simplest treatment is the reduction of sound reflections. Reflections cause echoes. Reduce reflections by installing curtains/blinds, furniture, and books in open bookshelves.
2. Standing waves - They cause bass to become inconsistent in the room, with some areas getting excessive bass and other areas getting weak bass. Standing waves are reduced by installing bass traps. As long as no dimension of the room (length, width height) is exactly the same as the other, you will not have any major issues regarding standing waves.
3. Soundproofing - Forget about it. It's very difficult and very expensive. Don't believe that you can soundproof a room by simply using acoustic panels. Absorption and diffusion will not work without decoupling and isolation of walls, doors, ceilings and floors.
In your case, just reduce reflections and you'll be fine. Don't totally eliminate all reflections, or you'll end up with a dead-sounding room that will sound weird.
Acoustic wall panels
Acoustic ceiling tiles
Full anechoic chamber
(Klipsch Audio Egineering and Testing Center)