I assume you use a laptop computer? Connect the projector to the laptop
using the RGB-cable usually supplied with the projector. Start both (With
older projectors and/or older versions of Windows, you have to start the
projector first, then the laptop. The sequence should not matter with modern
equipment.)

Start PowerPoint and switch to fullscreen mode (F5 or the little screen
symbol.)

If you do not see anything on the screen of the projector, find the key
combination on your laptop to display an image on both monitor and
projector. (Usually it is Fn plus one of the function keys marked with a
screen symbol; for my HP it is FN+F4.)

Plan enough time ahead (before your audience arrives) to rehearse this.

Best regards,
Ute