I'd start with thinking about these 3 areas:
1) basstraps - corners and where wall meets ceiling behind speakers
2) absorbers - to kill first order reflections from speakers - i.e. ceiling above, and to the sides (think of it as a mirror)
3) some sort of diffuser behind you - bookshelf with randomly stacked books is acceptable as a real world compromise....
other than that, aim for symmetry between walls, and ideally sitting at around the 38% mark of the room.... don't be afraid to experiment with speaker positioning too, especially if they are rear-ported. sub-woofer set up is a whole new area too....
good luck!