Windows + P (Duplicate or Extend Screen)

Hold down the Windows button and tap the P key.

This dialog box will appear:

Select one of the following:

"Duplicate" if you want to "mirror" or have the same image on both the monitor and projector screen.
"Extend" if you want to have two separate workspace screens that "extend" into each other.
"Second monitor only" if you only want to use the 2nd screen (such as just the computer monitor or just the projector).