Right, this is a huge area and I'm quite lazy so I'm going to fire off a quick 'Top 10' photoshop tools. You'll need to search these forums or google for further info on each. Also, I'm sure some people may well disagree with these points or have superior methods, this is just how I like to work!
Firstly, if you don't know already learn how to use layers and layer masks. These are the single most useful in photoshop. They allow you lots of freedom in applying changes to selected areas of an image. Also try and get an understanding of brush and layer opacity, and what the PS histogram is telling you.
Second, levels adjustments. Used to maximise the range of dark and light tone sin your image. Make sure you don't clip your blacks or whites. Can be used to increase contrats in an image without using the 'Contrast' PS tool, which often ends up losing pixel informaiton if pushed too far. Also useful in conjunction with layer masks for balancing the sky/foreground exposures in landscapes.
Third, curves adjustments. Essentially the same as the levels tool but with more control. An 'S' curve is often good for increasing contrast and giving punch to otherwise flat images.
Fourth, cloning. Very often comes in useful to remove distractions and imperfections from an image. An easy thing to use badly!
Fifth, channel mixer. One of several ways of altering colour and saturation (Hue/Saturation tool is also useful here). Also the best way of achieving B&W conversions by ticking the 'monochrome' box.
Sixth, transformations. Allow you to correct lens distortion, straighten horizons and other useful things.
Seventh, dodge and burn. Do not use the built in dodge and burn tools, they can do a lot of damage. Instead, create a new layer, with soft light blending and a 50% grey fill. Then paint onto this with low-opacity black or white paintbrushes to selectively darken or lighten areas of an image.
Eighth, noise removal. Try and get a plugin noise filter like Neat Image Pro or Noise Ninja, they do a fantastic job of removing noise from an image. Watch out you don't overdo it and soften your image too far.
Ninth, sharpening. There are several methods for sharpening an image, but a good general method is to use unsharp mask with a radius of about 0.8-2.0 and a threshold of 0-5. Apply sharpening only after resizing an image, as the last step in your workflow.
Tenth, filters. Ignore them, they are almost all useless (bar unsharp mask and lens correction, which has some tools for adjusting vignetting, chromatic aberration and wonky horizons)!
