Most Active Rain members probably are not Photoshop junkies. They simply want to use the "digital darkroom" to improve images they have taken of available properties.
I'm using Photoshop 7 in this tour, since 1. I am rather fond of it, and 2. Marcel Peterson asked about Photoshop 7 tutorials.
Here are the 3 basic skills that I think will put your photos a cut above most others found on your local MLS:
First technique: Adjust Levels
1. Hit - Image -> Adjustments -> Levels

2. The Levels dialogue box will open. Click and drag the center slider towards the left to lighten, toward the right to darken. Click and drag the sliders on either end until you are satisfied with the result. Click OK.

Second technique: Skew
1. Hit Select -> All to place a selection border (marching ants) around the entire image.
2. Hit Edit -> Transform -> Skew

3. Click to grab a "handle" and drag outward (or inward) until door frames, window frames, standing lamps, and other vertical lines appear straight. Hit Enter when you are satisfied.

Third technique: Sharpen
1. Hit Filter -> Sharpen -> Sharpen (or try the other sharpening tools)

In my opinion, just these three simple basic techniques will get you off to a fast start.
The crummy, totally uninspired image was shot in one of the rooms in my office with a Kodak V570