Make sure it has a PCI-E x16 slot on the motherboard. Then get a video card to fit your needs. Are you a gamer? Use it for video and multimedia? Or just general desktop use? Then start looking at a card that fits your needs and ensure you have a power supply that fits the requirements of the card.
If its an older motherboard it may not have PCI-E it may have AGP, which would limit you to older cards. It may also not have either, in which case it should have onboard graphics.