Yes, the download is free. However, to activate Windows 10 fully, you will need a valid product key. If you are reinstalling Windows on a PC that already had an activated copy of Windows 10, it will reactivate automatically upon connecting to the internet.
To ensure a smooth creation process, verify you have the following: windows 10 bootable usb download
Double-click the downloaded file. You will need to accept the license terms. (Nobody reads them, but click "Accept" anyway). Yes, the download is free