You have two main methods of finding out your serial number with a Windows machine.
1. You can run the WMIC Command:
(This may also show up as Powershell on your computer but will still pull the serial number through this command.)
2. You can physically find a sticker or writing somewhere on your computer that should have the serial number. This is generally located on the bottom of the computer near where you would see the battery. This information can also sometimes be found on the box.