First thing you will notice when you start up the Gimp is that there are multiple windows opened.  This is different than many other graphics programs that I have used in the past. Once you get used to it, I think it is technically better because you can make your image window as large as your screen allows.

The main window looks like:

One of the other major windows is the layers window. This allows you to draw on different layers so that you can put down a background image.  Then without altering the background, draw another image on top of it and work with that image without effecting the background.  Kind of like each layer is a sheet of clear plastic placed upon top of each other.  The window looks like this:

Of course you need your image window.  This is where the actual image is displayed and where you work on it.  A blank image may look like this:

If you accidentally close one of this windows and want it back, you must remember that The Gimp calls them "dialogs". I call them windows, because I am use to working in the MS windows enviroment.

To get your window, I mean dialog, back, go to the main The Gimp dialog and click the File menu, and the Dialogs submenu, like this:

Then just select the dialog you want.

Let's do something.  Let's work on a photo. Or maybe you just want to learn how to draw a line.