First, just make sure that the cables from the monitor (VGA) is connected to the GPU. If your GPU doesn't have them, make sure you buy an adapter. When you boot, you probably see small resolution (like 800x600 or something). But don't worry, it's because there are no GPU driver is over there. Then you would download or install the driver of the GPU from the CD that comes on the GPU or from the Manufacturers' website (either Nvidia or amd website, or manufacturers that you buy from like asus, msi, gigabyte, etc). After Installing you reboot and your system should adapt to the new GPU and use it.
Hope this helps and good luck