read the man page!http://www.glx-dock.org/ww_page.php?p=Tutorial-Customisation&lang=en
To open the configuration panel, <right-click> on Cairo-Dock, go to the Cairo-Dock sub-menu, and choose Configure.
Several categories are proposed in the left margin. The one that concerns us is the 2nd one (named "Appearance").
But first, we'll quickly go through the 1st category, that defines how your dock behaves.
Select the "Behavior" category; 4 modules are present :
* The first one, Position, can be used to modify where the dock is placed on your screen (and on which screen for people using multi-screen with Xinerama).