Mastering contrast with Levels
Contrast, or more specifically, a lack of contrast, is often the most noticeable fault in many pictures. This is partly because cameras are designed to capture images with a slightly lower contrast than was actually present in reality—and in doing so, they capture a slightly wider range of tones than if they were recording higher contrast from the get-go.
JPEGs are processed in-camera—which is why, when compared with a RAW file, they will always appear slightly more colorful, but you can always extract more tonal range from a RAW file. It just needs a little more work.
The best tool to begin editing any non-RAW image (such as a JPEG, TIFF, PNG, or PSD file) is Levels (Ctrl/Cmd + L or Enhance>Adjust Contrast>Levels).
Levels is used to adjust the tonal distribution in any image. You'll recognize this when you see the histogram—this is the same display that you'd see on your camera's LCD screen when replaying...
