Going to reinstall, how do I find my activation number

I'm going to take my laptop and make it our spare. Before I do that I want to wipe it and do a fresh install for several reasons. I know I'll need my activation key in order to reactivate windows, but I upgraded this pc from W8.1 to W10 and have no idea what I've done with the key. So my question is how can I get the key?

What information have I forgotten to give you?

If the computer came with Windows 8.1 from the factory, the product key will be stored in BIOS/UEFI firmware. If you try to install the same version of Windows 10 it will use that product key from BIOS/UEFI firmware and not even ask you to enter one. If there is not a product key in BIOS, then there will be a link on the page asking you for a product key that says, "I don't have a product key." Click that link, and as long as you are re-installing the same version of Windows on the same computer, it will retrieve the digital license for that computer from Microsoft Activation Servers when you connect to the internet and activate automatically.

Thanks guys for the answer, I'm going to read the guide, but I remember now about it being stored in BIOS. I don't do much with computers these days so I have forgotten so much.

NavyLCDR said:

... as long as you are re-installing the same version of Windows on the same computer, it will retrieve the digital license for that computer from Microsoft Activation Servers when you connect to the internet and activate automatically.

Just to add to that, the 'same version' means that the edition must match what you had before, The digital licence is only valid for one edition, Home or Pro. If it you had Home before you must install Home again.