Microsoft has Windows 10 available for download on their site here, for free. You will have had to have purchased it or upgraded from another paid version of Windows to be able to use it.
For detailed instructions on how to get this file from their site to a bootable USB Flash drive, this site has some simple instructions here.
First you will need:
- A blank USB Flash Drive with at least 4GB of space
- Another computer to download the Windows file
- A program to convert this file onto a bootable USB Drive
There are 2 HD formats for installing Windows. The latest (Windows 10) needs to be installed on a GPT-formatted HDD. See the above article for details.