First off, if you have a hard floor like cement or wood or tile...get an area carpet.
Otherwise, you need to use absorber panels.
Generally acoustic studio treatment requires 3 devices....bass traps, absorbers, and diffusors (heads up, you won't need all of them).
Bass traps, trap bass in the corners of rooms to prevent standing waves (spikes in bass volume).
Absorbers simply absorb sound, especially the higher frequency, to prevent reflections and echos.
Diffusors diffuse sound. This prevents abnormalities and phasing issues with reflections, while not killing them totally (like absorbers do).
I recommend you research all of them, to see what you want.
BUT, long story short, if you just want to kill the echo, all you are going to need is absorbers. If you want to do highly critical listening, like mixing and master with studio monitors....then you are going to want to spend the extra time for diffusers and bass traps.
The wonderful thing about sound absorbers is that they can be made quite easily, cheaply, and be custom styled to even improve the room aesthetics.
Generally they are 2x4' panels. You make the frame out of wood, like 2x2 spruce, then you put a single bat of home insulation in the frame, then wrap in with the fabric of your choice. You can find lots of tutorials online...but as long as you have insulation in fabric hanging on your walls, you are good to go.
You won't have to cover the entire wall, maybe 25-30% of the surface will help.
Here is a tutorial, but I encourage you to investigate all sorts of designs.