I'm not sure anyone could provide you a full tutorial on how to rescue over-dark & wrongly-lit photographs. This one suffers from both those issues & also a third… it's not of high-enough technical quality to rescue.
It's too noisy. If you try to lift the dark areas, there's not enough detail in them.
For a beginner, this type of rescue operation is often best performed by a dedicated HDR app, such as Aurora HDR. It will attempt to lift the dark areas without killing the bright areas… however, in this case there's not a lot that it can do to fix this shot.
Here's a two-minute attempt, to show you how little detail can be regained.
The only way to have fixed this shot would have been to not have almost all the lighting behind the subject.
Following Billy Kerr's answer using Gimp itself, here's an alternative method using Curves. It's similar to using the Shadows/Highlights feature, but allows for finer control. The same caveats apply, the image is too dark & too noisy to fully rescue by this or any method.
Here I've over-pushed the settings to make the result clearer to see…