You can pick any gun you like, but for this tutorial, we will be using a revolver

spawn in your gun facing left and then spawn in a human facing right.

then put the gun in the humans hand by hovering over the hand then pressing f (Activate object under cursor / currently holding button)

then aim the barrel of the gun towards your head, just like the image

Press your activate object under cursor / currently holding button with your cursor hovering over the gun to shoot it

You killed yourself, nice