If it is an external webcam, it should have come with a disk with the drivers on it. When you plug it in, the system should go ahead and recognize new hardware and take you through the New Hardware Found wizard. Windows 7 also has a comprehensive database of drivers for a lot of peripherals, it might not even require a separate driver installation from said disk. Of course, manufacturers usually put on that disk not only the drivers, but also some utility programs for that peripheral.
If it is an internal camera, you need to visit the manufacturer's website, navigate to their customer support page, downloads, and look for your system. There should be a Drivers and Utilities page that allows you to download and install the files needed (driver and utilities).