The degree that you use the two sliders will vary depending on the photo you are editing, though do note that editing a photo too much can make it worse. You should also remember that some photos cannot be improved, so don't think that this easy photo edit will always work.
If you have a photo that looks too dark, moving the Brightness slider to the right will lighten it, though be aware you will also reduce the contrast.
If a photo looks flat and lacking contrast, moving the Contrast slider to the right will give it more punch. You need to be careful not to adjust the sliders too much as the results will look unnatural. With a little care, this is an easy photo edit in GIMP that anyone can use, regardless of experience.