CTRL-b, " - Split window horizontally (i.e. split and add a pane below).
CTRL-b, % - Split window vertically (i.e. split and add a pane to the right).
CTRL-b, UpArrow - Move to the pane above (I prefer to rebind this to CTRL-b, k to be similar to Vim).
CTRL-b, DownArrow - Move to the pane below (I prefer to rebind this to CTRL-b, j to be similar to Vim).