Acoustic panel for room treatment

Hello,
My audio room size is 15x30x8.
It is in half basement, as seen.
https://www.dropbox.com/s/5v394kjl2b99nx6/IMG_1003.jpg?dl=0

I am thinking of treating the room with some acoustic panels.
Any suggestion? How many? How big? Brands? Where in the wall?
Thanks.