After you plug in your second screen, please follow the following steps to configure them properly
1) Right Click Desktop and click Display Settings
2) You will then get a new window that looks like this. Click on Identify
3) You should then see numbers appear briefly on your monitors. You can drag the displays in the settings window to match what you have physically setup. Eg, if after clicking on identify your left window briefly shows the number 2, but the settings show the left monitor as number 1, you can click and drag 2 to the left and click apply
4) After you do that, your dual monitors should be setup. You should now choose which screen should be the Main Display. To do that select the screen that you want to be the main display and scroll down and checkmark Make this my main display. Any programs that you launch will automatically open first in your Main Display and you can then drag them over.