Of course, you can buy a key from Microsoft online, but there are other websites selling Windows 10 keys for less. There are many ways to get a Windows 10 activation / product key, and they range in price from completely free to $309, depending on which flavor of Windows 10 you want. You can skip it, but eventually, Windows will start alerting you that your install isn't activated. During installation, Microsoft asks for an activation key.
Once that's done, you can boot from your installation media and load Windows 10 onto your PC. Whether you've paid for Windows 10 already or not, anyone is allowed to download a Windows 10 ISO file and burn it to a DVD or create installation media on a USB drive for free. Getting hold of the Windows installer is as easy as visiting.
