How to install os on new hard drive with no previous os

So I just bought all the parts for my new pc im building and was wondering how i would download windows 8.1 if i have no previous os on my hard drive?

Basically if you have an install disc just put it into your ODD and select boot from that drive in your BIOS. 

If you don't have an install disc or an ODD then you will need to download Windows and mount it to a USB flash drive. Then insert it into the PC you want to instal it on and in the BIOS tell it to boot from that drive.

download it on another computer.

Thanks! Helps a lot