Photoshop is my first choice for post processing. It is so feature rich that after 10 years I am admittedly still learning this software. I started with PS Elements and it can amazingly do a lot. I migrated to Creative Suites at version 3 and now use 5.5.
This post is about Levels adjustments, which is always my first step.
I always begin with a levels check, no matter how the photo looks. Levels is in the Image pull down menu, Image>Adjustments>Levels. The short cut is Control L in windows and Command L on a Mac. It pulls up a histogram that basically indicates the Exposure adequacy.
Here is an example of an image:
and it's Levels histogram
A perfect histogram would be a bell shaped curve with the most data (black) in the center. As you can see, this image has too much dark and not enough light hitting the surface. Moving the white slider, which is right above the number 255, to the left just to the point where the data is beginning to appear (black blob), will vastly improve the image. It has the effect of making everything lighter, both darks and lights. In this case I nudged the black slider as well and usually find that I need to nudge both. Depending on how the photo looks after these adjustments, I sometimes need to move the middle one as well. The middle slider adjusts mid tones. If you make a photo too light, it often takes on a smoky look. This midtone slider will improve that.
Here is the histogram moves I made and the resulting image. You can see the lights are brighter and the edges of the leaves become crisper, more defined against the dark background. If I had moved the white slider even more, I would have picked up some of the detail in the background but don't want that. It subtle and very natural looking but markedly improved IMO.