GIMP is a great free program but how do you learn to use it? For me, I used trial and error, and played around with it until I got to about the intermediate level. I tried every tool and filter, and looked at other menu options.
If you don’t want to use the “stumble around” approach, try the GIMP tutorials.
One very important concept that I didn’t immediately grasp was the concept of “Layers”. Think of layers as transparencies that you can lay one over another. You can work on one layer and add it to another, moving and adjusting layers until you have assembled the picture you want, flatten the image and there’s your picture.
Here’s a YouTube tutorial on GIMP and layers: