You can divide any window by right-clicking along its borders. Your cursor will change to the resize symbol so you know it's in the right place.
Then, you can choose to create a vertical or horizontal split in that panel. You can actually split any of the available windows by dragging the dotted split line, and clicking when satisfied.
Joining panels back together is also done by right-clicking, but it should be noted that you can only join two windows which share sides. For example, you could join the 3D view window above with the new 3D view just created, but you couldn't join the 3D view window with the Outliner and Properties Shelf windows on the right. Those two windows would first need to be joined themselves.
Alternatively, you can also hit CTRL+ALT+Q to toggle quad view, which will automatically split your 3D view into four panels - one for each of the three orthogonal cardinal directions, plus the view you already had open.