Tools -> color tools -> Curves.
For curves, just like in photoshop, you pick the color you want to change on your image, and the curves tool wil mark out where that specific pixel is on the histogram. Drag the curve up to remap the value brighter, or down to remap the value to a darker one.
If you want a more in-depth look at using curves, i suggest reading any photoshop tutorial on curves. The same methods apply in Gimp, the buttons are just in a different spot.
Also, I use Brighness/Contrast and Levels. I suggest simply messing around with those two, because they are VERY straighforward once you have a chance to play around. The specific order I do things in is:
- Levels (to adjust dynamic range and the gray point)
- Curves (to touch up specific areas and ranges, change white balance)
- Brightness/Contrast (a quick and effective way to readjust the brightness in the image without spending a huge amount of time in curves and levels)