in photoshopCS (at least) you can find curves in Image > Adjust > Curves (in CS its the 5th one down from the top).
I find curves much easy to use for colour corrections than anything else. Its mostly a matter of findg a spot you -know- is supose to be white (or black) then shift clicking with your tool as the eye drop, and putting a place on it. In the corner of the screen there will be a box with numbers for RGB... and adjustsing the colour channels (not RGB as a whole, but Red, Green, Blue individually. Its usually just one channel that's off. My scanner tends to give things a red/pinkish cast to everything, so I usually have to adjust the red out.
White RGB is all all numbers being 255. Black RGB is all colours being 0. Anything in between all those like 155 << will give you shades of gray. I'll work on some screenshots now to try and show what i mean... ( i know sometimes my speaking only makes sense to myself. >_>)
i think my screenshot may make less sense... but meh, since i said i'd post... <_<
http://i78.photobucket.com/albums/j103/ ... sshelp.jpg