I'm not sure how you should cover the walls, but you can buy some cheap foam to start with and just start playing around until you feel like it sounds decent. If you want to go a bit pricier get some Acoustic Wall Panels or just build your own. Cover what you can but don't over do it. A bit of color can be good, just be aware of it. Carpet looks thick so that's already good. Get some Bass Traps for sure. Two in the corner behind you should do the trick. Buy these these for those monitor stands if you don't have them.

If you want to get technical, do some research here.
You can also measure you room with this, but you will need a measurement calibrated mic. Those get pricey.

Good luck!