You need to determine if the PC came with an OS. Read the materials that came with it - were any disks provided?
Then look on the case for an official MS COA sticker to see if an OS is installed. The sticker looks like this:
With that sticker you can install the licensed version given on the sticker from the link here, using the 64 bit installer, following these same steps to get a perfect install: Clean Reinstall - Factory OEM Windows 7.
If you bought a PC without an OS then you'll need to buy one or install a free Linux distro.