My laptop is on and plugged to the monitor, but

Make sure you have power turned on the monitor and the laptop. Hook up the external display to the laptop with the appropriate cable. Look at all the F keys on the laptop and you will find the F key with the symbol of two monitors on it (mine is F4). Press F key with the symbol of two monitors and you will start to see pictures on the external monitor rather than the screen of a laptop. Its very rare that you will need a driver but if you do require driver go to google and type the company name of the monitor and insert word driver infront of it. I hope this helps and good luck. Please vote accordingly and leave a response.

You have to switch your laptop to a different display mode. Usually it's done by pressing Fn key and F8 - look for a description near this key showing rectangle / rounded rectangle, or text "CRT / LCD".
Usually there are 3 modes:
LCD (internal laptop display) only
LCD + CRT - the picture shows on both displays
CRT - the picture shows only on the external display, allowing you to set a different screen resolution, equivalent to the external monitor's optimum resolution.