It depends on the set-up of your keys.
For instance, some laptops will have extra functions added to some keys.
The keys alone will be what the keys are for. These are usually F1 to F12.
On some of these keys, there are secondary functions printed in white. They may require the CTRL key to be pressed and held down when pressing those to activate the secondary function.
On some of these keys, there are more functions printed in blue. They may require the FN key (which is usually blue as well) to be pressed and held down when pressing those to activate the secondary function.
Anyway, when pressing the PRT SC key... the screen will be copied. Now, go to MS Paint and then press CTRL+V to paste it. That should work.