I usually works with two screens, the screen of my laptop and another external screen. It makes me work more effectively because I can have two windows open at the same time.
Using only the first monitor
It might be easier for someone using only the external monitor. To do it, you just have to close the lid of the laptop a bit, not totally, then the O.S. will detects that the screen of the laptop has turned off but the external screen not. So, it will shift everything of the screen of the laptop to the external screen.
It is a bit tricky at beginning. Try it several times and you will get it.
Switch back to the two screens mode
If you want to switch back to the two screens mode, an easy way is using the monitor preferences.
1. Click on System Preferences
2. Click on Displays
3. At the button left, click on "Detect Displays"
Then, the O.S. will detects that you have two screens turned on, and it will switch back to the two screen mode.