There are two basic tools in Photoshop for correcting exposure:
Levels dialog - used for maximizing tonal range and rough brightness adjustments.
Curves dialog - fine tuning the tonal balance and contrast.
When viewing the histogram of an image, if the left side (shadow) ends abruptly this means that the image was "clipped" and that the darkest areas of the image will be rendered as solid black. This happens when the darkest areas of an image are below the image sensors detection range.
The Levels dialog is a quick and easy way to adjust the exposure of your image. Open the Levels dialog by selecting Image...Adjustments from the menu. From here you can either do Auto Levels or Manual Levels. The Auto Levels command will automatically stretch the histogram so that there is a better distribution from shadows on the left to highlights on the right. If you prefer to have more control over the results, you can use Manual Levels. With Manual Levels, you adjust the sliders under the histogram until you get your desired results. Typically, when adjusting an underexposed image you will adjust the middle and right sliders.