Think of it like paint with a few exceptions. Left clicking is your action button, while right clicking brings you a menu of sorts.
Here's the screen that is similar to paint's:
You can find out what each of the buttons do by just playing around with them. However, below all your buttons and before that divide, you have foreground and background color (in that order. you can switch them too by the arrow button above them). A brush (double click it to change it. Play around with them sometime. There's some pretty cool ones in there, with one for almost every job), pattern, and gradient. The pattern and gradient are used for various things, but you might not and don't really need to, but it's cool to do sometimes. Then there's the section below. You can use this to refine what you want to do with your tools. Sometimes it's helpful, sometimes it seems useless, but I'll let you be the judge on that.
This is your palate, and, besides the buttons and color changings, you'll select and do everything else here (however, you don't have to use it at all, more later on that). Right clicking will give you this:
These options are fairly self explanatory.
Then there's edit:
This might be slightly confusing, as to copy to an external program, or paste from an external program, you'll have to use the last two options. Other than that, everything else shouldn't be too hard to figure out on your own.
Then there's this:
Here you can crop images, resize and rescale image (Canvas Size and Scale image respectively) and flatten images, when necessary.
If you're too lazy to bring up the first screenshot that I showed, you can use the tools option to do the same thing. I just mentioned the other before, as this is the first one that you'll see. There's also more options here, but you probably won't use them all. No screenshot here as there's not much to show. You can work this out on your own.
Dialogues is similar to Tools, so I'll leave that one at that.
Then there's Filters and Script Fu. They're basically the same, the difference is how they're programmed. Have fun and check out each option. It's fairly likely that you'll find something that you'll like and could possibly use. These are basically like Photoshop plugins (You can also import these, but only if you have photoshop. However, I don't have much use for them. I've tried it before, but prefer not to do so).
And if you saw the last option in my screenshot, ignore it, there's nothing to
see there

.