Things in the Gimp that I've found particularly useful, and a major mind-change for us MS-Paint self-taught hacks:
1) The built-in Help system is very well done; wonder what a button/feature does? Check the help!
2) Selection tools are powerful - select by lasso, colour, contiguous regions, etc.
- When you use them, have a look at the 'Select' menu of the image menu; being able to manipulate exactly what you want in an image is key.
- Also, note, in the Edit menu, that you can 'stroke' a selection - this lets you outline things, create shapes, etc.
- You have to play to understand the power. Consider selecting a region and then 'floating' (Select menu -> Float) it to its own layer.
- Change opacity, etc.
- You can reorder layers by dragging them.
4) Script-Fu and Filters
- Play with them to understand what they do.
I'm starting to realize the power of the 'History' view where you can jump back/forward many steps at a time, and there's so much more to learn...but it does get easier with time
Good luck - ask questions!