You'll need to go into the BIOS to deactivate your onboard graphics card, which you usually get into by pressing the DEL key as soon as you first see text appear when your PC is on. From there search for an option to disable the onboard graphics card (but don't change anything else unless you know what you are doing).
Most of the time the onboard graphics card is disabled when you insert another graphics card into the motherboard, so you might not need to do this.