Well first of all you will have to learn the path tool... Thats basically the hart of anything you do in GIMP except if you wanna do a brush SPAM art work.
So yeah... Master the skills of path tool, and you're on your way to success. Actually its Paths tool AND the use of layers.
Couple of hints for path tool:
1: When you are using path tool and you've already drawn a line or a shape, don't get cared when you click an other tool, and your line goes. You can click the "Open the paths dialog" button that is on the "Layers, Channels, Paths, Undo, Palettes - Brushes, Patterns, Gradients" toolbar, and there you can find each of the paths you have drawn recently. (The paths will only be there from when you draw them until you close GIMP).
2: If you want to color the area inside the shape you've drawn with path tool, you will have to click the "Selection from Path" button. When you press it you will see little black lines going around at the edge of your shape. (If you can't see it press: Ctrl + t).
This enables you to only color the shape, and not the area around it.
3: If you want to color the area around it you will have to color the area around it, then do the same thing that I told you in Tip no.2, only you don't color the shape, but you press Delete. You will see what happens...
4: If you wanna color the shape and the background, then you will have to make an other Layer, (You can also find this button on the "Layers, Channels, Paths, Undo, Palettes - Brushes, Patterns, Gradients" toolbar) choose whichever you want, do completely what I've told you in Tip no.2, press Ctrl + Shift + a (this will stop the black lines going around your shape, and allow you to color anything not just the shape you've made). Now click on the other layer, and just color it to whatever color you want.
These are just few tips... But you can do much more than that with the Paths tool.
But if you watch some tutorials on YouTube and read some from the Tutorials board you will get used to it kinda fast.
And the most important thing you need for drawing is creativity.
