There are many factors involved with acoustic treatment; budget budget being a big one. You will not be able to turn your bedroom into a recording studio. Having said that, you will notice a difference with every foam panel, bass trap, diffusor, etc. you install. I would recommend light treatment with some foam to help with early reflections. You'll definitely notice a difference. The improvement will inspire you to build bass traps which actually make the biggest difference in a small room.
Be prepared for overwhelming advice about how you actually need to rebuild you house to get the frequency build up at 400Hz under control (I'm exaggerating...slightly). I would suggest making small changes at a time instead of spending a lot of money treating a room you're not even sure is worth it.
What I tell people is, "Once you hear a properly treated room, no one needs to explain why you need it. " Same with studio monitors but thats a different topic.
A multi-use room always involves compromises, but with some research and careful planning, you should be able to improve things dramatically. Let's start with a few basics...
1. Your listening position/desk should be symetrical between the walls on the left and right.
2. Broadband abroption (not foam) at first reflection points - side walls, ceiling, and rear walls.
3. For a small room like yours, you can't over-do corner bass traps (wall-ceiling corners are often a good choice because they don't take up any floor space).
Of course, the biggest compromise will be mainating this as a usable bedroom. How do you feel about a sleeping bag on the floor every night?