PC World has covered this process many times before, so rather than repeat it here, I'll direct you to Patrick Miller's

Next, you'll need to . It's available in 32-bit (x86) and 64-bit (x64) flavors. Most modern PCs can run the latter just fine, so that's what I recommend unless your system is more than 3-4 years old. Whatever its age, be sure to check Microsoft system's requirements. Likewise, make note of the product key: TK8TP-9JN6P-7X7WW-RFFTV-B7QPF.

By far the easiest way to install Windows 8 on that new partition is via a USB flash drive. I described this process last year in so those instructions should get you squared away.