Here's the clean install tutorial:
Clean Install Windows 10

You won't need a product key so long as you install the same edition (IE: Home or Pro) that was on the computer before.

But if you want to know your product key, if you have:
Windows 10 Pro - VK7JG-NPHTM-C97JM-9MPGT-3V66T
Windows 10 Home - YTMG3-N6DKC-DKB77-7M9GH-8HVX7

You can check for yourself with Showkey Plus: