You won't need a key. Since you already have Windows 10 on the computer, a digital license is stored on the Microsoft servers and will be retrieved and applied when you reinstall Windows 10.
You should download the latest available which is 1903 18362.239 using the Windows 10 Media Creation Tool.
Click on the Download tool now button and save the Media Creation tool to your Desktop.
Expand Using the tool to create installation media and follow the directions to create a USB flash drive (8GB or larger).
Disconnect any drive except the one you will be installing Windows 10 to.
Boot the flash drive.
During install when you get to the window where it shows you your drive, delete all the partitions so it is a unallocated disk with no partitions then click Next. The installer will create the partitions it needs and format them.
Install should only take 30 minutes or so.
If you want to use a Local account instead of a Microsoft Account, don't have the computer connected to the internet during the install. You can connect it after the install completes. I recommend using a Microsoft Account but the choice is yours.
Once it finishes, power down, reconnect any disconnected drives, power up, connect to the internet then do an immediate Windows Update. Once there click the Check for updates button. That will get you up to the latest service level which is currently 18382.387.
Edited by Ztruker, 01 October 2019 - 03:30 PM.