There's also Paint which is most likely already on your computer.
If not;
PaintGIMP is really good. It's one of those programs where you can use tutorials to learn the basics and mechanics of it but still experiment with the way you want.
It's also okay to request new programs to use event hough you have GIMP already. There's no harm in that, your just broadening your horizons. I know plenty of people who have GIMP, then photoshop ectr. then barely use one over the other.
But like everyone else recommended there are some really good tutorials on deviant art and even here on Gaia that I've seen.
There's this other program I've used my computer teacher recommended it. I think it's pixel paint or something.