You mean Gimp the painting program? Well there is a lot of ways I make a picture on there. The usual way I do it is that I click file> new> enter in the size of my new picture in pixels> then I use the tools in the tool box like the paint brush, spray paint, pencil, dodge/burn, blur/sharpen, and etc.
If you like to know more, I suggest checking out the tutorials page: http://www.gimp.org/tutorials/
Explore around, mess around with it, scribble with it, practice all the features. That's how you can get used to it. It's a fantastic painting program and I use it on a regular basis. For example, here are some pictures I made with it:
Daydreaming in Class
Princess Bubblegum
Self Portrait