No clue

[Redrobes]

Maybe if its your first time with a new app its a good idea to just play around with it and get to know some of the real basics first. Just paint some lines and then spray some stuff, use a little delete and move on until your at least confident with the user interface and basic features then at least your not trying to hit all the difficult stuff and simpler stuff all at once. I don't know Gimp all that well but its an app that has huge potential and with those sorts of app they have features which are powerful and sometimes difficult to get to know how to use all mixed up with the plain easy stuff. Layers are not the simplest thing to get to grips with for a first time use of Gimp and might be off putting. If you have thick skin and patience then sure it can be figured out ok but don't throw the towel in if it starts looking too difficult cos you should progress in stages and you would get more out of it in the process.

Not knowing Gimp too well I am unsure what your specific issue is. Normally you select the right layer and then use an effect like a brightness / contrast to change that layers image. But it might be that you need to fade in a white backdrop or maybe multiply it with something less dark - all could give the same effect. Post up images if you get completely stuck and we can see whats happening.