The most expensive monitors in the world won't help you if your room is lying to you. Acoustic treatment is about managing reflections and bass build-up so you can hear your music accurately.
1. Bass Traps are Priority One
Low frequencies build up in the corners of a room, causing "mud" and making it hard to judge your kick and bass. Place thick absorption (like rockwool or fiberglass panels) in all four corners, from floor to ceiling if possible.
2. Managing First Reflections
Sit in your mixing position and have a friend slide a mirror along the side walls. Wherever you see the reflection of your monitors in the mirror is a "first reflection point." Place absorption panels there to stop the sound from bouncing directly into your ears and blurring the stereo image.
3. The Cloud
If you have high ceilings, reflections from above can cause phase issues. Hanging an absorption panel (a "cloud") directly above your mixing position can significantly improve the clarity and focus of your sound.
4. Diffusion vs. Absorption
Don't just deaden the whole room. While you want to absorb the "problem" reflections, a completely dead room feels unnatural to mix in. Use diffusers on the back wall to scatter sound waves and maintain a sense of "air" and life in the room.