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.
We need more info here, tell us your FULL specs: CPU, Motherboard(with no. series or model), RAM, Power Supply Unit, Monitor, Operating System, Budget?