Carpets, curtain over the window. Bass traps in the corners A quarter of the size of any offending standing waves you find and some acoustic panelling behind you. Obviously glue would be a good alternative to screws or nails. Or some kind of velcro tape, not sure that would be strong enough though.
It could be expensive unfortunately. I haven't been able to treat my room due to lack of money !
It might not make it perfect but it will help a lot.
Putting the desk where you have it now will probably create an uneven stereo image because the side walls are not the same distance from the desk/speakers.