Adding a new resizeable area is done by grabbing the diagonal lines ("little ridged thumb grips") that are present at the corners of all areas and moving them to the desired direction, to obtain an "up-down" or a "left-right" split.
Once created, each area can be assigned to a different type of editor.
An area can be removed by "coalescing" a neighboring area onto it. This is done by grabbing the same handle and, while holding the mouse button, moving the pointer to the center of the area that you need to remove.
Here's an animation of what you are trying to achieve
Hope this helps.