Hi Jean,

It depends what you are trying to say.

For general advice or in a set of instructions ( which apply to everyone) you will use the infinitive - 

Ne pas oublier de se laver les mains en sortant des toilettes = Dont forget to wash your  hands after you’ve been to the toilet

Ne pas se pencher par la fenêtre = Do not lean out of the window

Prendre le billet au guichet svp = Take your ticket at the counter please

But if you are making a direct command to someone it will be the imperative.

Hope this helps!