Once you've opened a digital photo in GIMP that you think could be improved by increasing the contrast, go to Colors > Brightness-Contrast.
The Adjust Brightness and Contrast dialog is pretty simple and easy to experiment with. One of the nice features of the dialog is the Edit these Settings as Levels button. After making adjustments using the Brightness and Contrast sliders, you can press this and see how you could have produced the same effect using the Levels feature. This can be a great way to introduce yourself to a more powerful image editing tool. On the next page we'll see how the Brightness-Contrast feature can be used to make an easy photo edit.