Set the option for the location of the iTunes Music folder to your external drive in the iTunes -> Advanced -> iTunes Music Folder location preference to the appropriate place on the external drive.
Note that this will only affect where future imports and purchases go. If you want to move your existing content there, the easiest way is to just drag the entire iTunes folder (the _entire_ folder, _not_ just the iTunes Music folder) to your desired location. Then hold down the Shift key while launching iTunes. You'll be given a dialog box where you can select the iTunes library you want to use. Navigate to and select the iTunes folder in it's new location. The your existing content will be on the external and all future imports and purchases will be stored there as well. This procedure assumes that all of your tracks are contained in the iTunes Music folder. If they're scattered around your volumes, moving them becomes much more complex.
Regards.