There may be more than one way, but this is the way I do it.
(This is using Photoshop 7.0, but I'm sure 6.0 is the same)
Start with a color pic, because if it is black and white to start with, you can't adjust the color. (Sometimes I can mess with the color on black and white...it's weird. Just try both I guess)
Then go Image-adjustments-hue/saturation. Turn hue and saturation all the way down but leave lightness. Now it looks black and white. Desaturate will work also, but sometimes after I use that, I can't mess with the color.
Then I open a real sepia picture to compare it to.
Then adjust the now black and white pic: Image-adjustments-color balance. Usually just turning up the red will do it, but you might have to play with the other colors to get the right color you want. I just mess with it until it matches the real sepia pic.