First you have to find out what video card you have.
You can right click on your desktop and select properties. Then go to the settinigs tab. It will show you what type of video card you have. If you want more info you can click on the advanced button. There it will tell you more stuff like how much memory your card has.
After that, go to the manufacture's website and download the drivers related to your video card.
If you still can't find out what video card you have try downloading this to see if it can identify it.
Download EVEREST Home Edition 2.20 - filehippo.com