Once it's plugged in, go to your Displays System Preferences pane and you'll see on the primary monitor (the one with the menu bar and Dock (if it's on the bottom of your screen)) a selection for "arranging" the monitors.
You'll have to play around with this to get it just right for dragging windows across the two, but eventually you'll get it, and your computer will remember the setting.
Every time you open the Displays System Preferences pane, it will open in the centre of BOTH monitors, to allow you to set resolution and desktop picture settings for both.