> where do you start? <
Bruce gave you some good advice, and you'll find a lot more here in my Acoustics FAQ:
www.ethanwiner.com/acoustics.html
The key is as much bass trapping as possible. This is not an option, it's a minimum requirement.
Understand that it's not possible to make any smallish room perfectly flat - not that speakers are ever perfectly flat either! But the room is always much worse than the speakers, and you should aim to get the room as flat as its size and your budget allow. Ignore this and you will always be guessing as to what's really happening in the low end of a mix.
--Ethan