Having two screens is great to get more work done quicker. In fact, in certain professions, having a secondary monitor is a necessity.
In Mac OS, like most operating systems that support multiple displays, you have the ability to specify which is your primary. The primary display houses the Dock and menu bar and is also where windows will open by default.
Changing you default monitor is easy. In the Displays panel of System Preferences, click on the Arrangement tab. Here you can set the relative positioning of your screens. By dragging the menu bar from one screen to an other, you change it as your primary display.