Many people just starting out tend to get intimidated by Gimp's interface. In reality, it's not that difficult.
The Gimp User interface is broken up into 3 main areas. 1. The Main window.
This is the window that you'll be staring at the most, it's the main window you use to open, view, edit, and save images with. Once you load an image into Gimp, by using “File”, “Open...”, the main window will change and display the image as the focal point. At this point you can begin editing your image, or you can load further images, which will spawn new main windows. This window also contains the menu bar. In it, you will find more advanced options, such as image tools, layer tools, different configurable plugins and filter, as well as the manual (under “Help”). 2. The Toolbox window.
This is the window that houses all of the image manipulation and modification tools. As the name implies, it houses all of the 'tools' you will be using to modify your photo, or paint new ones. Normally, there are 33 or so tools in the toolbox, ranging from painting tools like brush and airbrush, to transformation tools like scale, move, or rotate. The lower half of the "Toolbox" window is for changing the settings of the tools. For example, you can change the drawing opacity, scale, and brushes of he "Paintbrush" tool in this menu. 3. And finally, the Layers and Channels window.
This is the window that lets you manage all of the information available about your current image, including the different layers you have, their respective color channels, and other things like paths or brushes. The default window also contains an undo history list, as well as a list of your currently installed brushes, gradients, and patters that can be used in conjunction with various tools in the "Toolbox".
And that's it! Just those 3 windows, each with their own use. In the next version of Gimp, there will be a "Single-Window Mode". Essentially, with that feature turned on, all of the windows will be combined into one, single, window. However, none of the basic tool locations should change.
Oh, and one final note. If you couldn't tell, I am using Gimp on a Mac, but it's not like that changes anything.
Also, lisanet provides a few themes to chose from the get-go, this one happens to be "Pro-app".
Windows users can check out link or whatever for themes.
Gimp on Linux uses the systems default theme if I'm not mistaken.