Log into your Chromebook. It doesn't have to be the just released Chromebook ARM. The free storage deal are also available to you if you're using a Samsung Chromebook Series 5 550 or the desktop Samsung Chromebox Series 3. If you're using an older Chromebook, such as a Cr-48, Acer AC700, or Samsung Chromebook Series 5 you're out of luck. You also can't get these offers if you're running Chrome OS on an ordinary PC—darn it!
Next, if needed, let your Chromebook update to to the latest edition of the Linux-based Chrome OS. To get the Google Drive deal you need to be running at least Chome OS 23.
All done? Then click on the Google Drive offer page. Google will then check to see if you're using a real-deal Chrome OS device, and a few minutes later you'll have your 100GBs of free storage for the next two years.
Don't get any cute ideas about getting 200GBs by buying two Chromebooks. You can only get a single free 100GBs of storage per Google account. You also can't transfer the storage space to a buddy's account.
What happens to your files at the end of two years? Nothing. They're still there and you can still use them. Come that day to add more files to your Google Drive, you'll need to buy more storage. At this time, Google will let you buy up to a mind-binding 16 Terabytes.
Once you have the free storage you can, of course, use it on any of your devices with a Google account. So, for example, you'll also have access to all your files on your conventional PC, a Nexus & tablet, whatever.