Depending on your PC this may be impossible as the motherboard may disable the onboard video as soon as you try to use an external video card. This is most common with older chipsets that have AGP video onboard and an AGP expansion slot. Usually it is recommended just to get an external video card with multiple monitor support (many low priced ATI/Nvidia cards support this)
No one is going to be able to provide specifics without knowing the specs on your computer, but the first place to try to mess with settings is to go into your BIOS at boot and see if it has options for picking/enabling your on board video.