Tool Options Window
You can open up the tool options dialog by going to Windows > Dockable Dialogs > Tool Options (first selection)
Templates Window
Since all of us here at WA are web designers and will most likely be using GIMP to create images for our websites, I think pointing out the templates window is essential, because it is in this window, you can select what size you want your image to be, such as 640x480, 800x600, etc.
To open the templates window, go to Windows > Dockable Dialogs > Templates (towards the bottom). It will look something like this:
Spend sometime getting familiar with the navigation of GIMP. It looks daunting at first, but once you start to become more familiar with it, it will be easier.
The last page I will discuss is the image window, which of course is the main reason for learning GIMP, because without an image, what is the point?
-Kathy