The first thing you will need to do is identify the wires in your wall. It sounds like you do not have a neutral (a return path to your panel).
The VPT24 does require a neutral because of its display and features.
Most switches do not, in fact a common mistake is incorrectly wiring the timer by excluding the white lead (wire) which is necessary for it to function.
So, the hot wire from your electrical panel (supplies power to the device) goes to the black lead of the VPT24
The neutral goes to the white lead.
The wire that carries power to the light you wish to operate attaches to the red lead.
Make sure it is grounded and that should do it.