When I first used GIMP, I used it almost exclusively for signature-making, so I learned about the program and how to use it by trying out a ton (and I mean a ton x_x) of signature tutorials on deviantART and graphic-centered forums. Then, after I had a better grasp on the program's features, I moved onto things like wallpapers, large pieces, photo manipulations, and etc. I read tutorials for those as well, but because I knew of the basic tools from using GIMP for sigs, I was able to follow those tuts correctly and then get to a point where I didn't need tutorials to tell me what to do anymore. Actually, at one point I got so comfortable with GIMP's features that I could even follow Photoshop tutorials and understand where everything was despite the difference in layout and filters.
Of course, I can't recommend this method for everybody, as people obviously learn differently, but this is what did the trick for me. Honestly, I find that experimenting with programs yourself can only get you so far; you do need to read some guides here and there, and for that I would recommend the official GIMP handbook (which I can link you to, if you're curious!). The thing's ginormous, so digesting it all in one day or a short span of time wouldn't be recommended, but if you really want to work at becoming good with GIMP and maybe understanding as much as you can about it, that's the best route imo. Otherwise, Ctrl+F is your buddy
Hope this was helpful :-]