Piece of cake. Assuming you are running OS X go to system preferences and select "Displays". One of the tabs should be "Arrange", just drag the little menu bar in the picture from the monitor you want to be secondary to the primary. You can also drag the monitors around if you want to rearrange which one is on the left/right.

Edit: the procedure under OS 9 and earlier is very similar, just open the Monitors control panel and dig around until you figure it out.

-Mike