To get started, head to the Security checkup that Google provides for your account. You'll need to verify your backup information, including phone number, recovery email address and backup security question. These fields must be done for this portion of the security checkup to be considered complete.
Now you'll manage to check your recent activity. This is all devices that have recently signed in to your account. If you have any mobile devices you have attached to your Google account, you will see them in this list. If something looks wrong, you'll be prompted to modify the password on your account to make sure the device in question can no more connect.
This next section might be a long one if you've used Google to log in to many devices or websites. Each device and site will be listed with a Remove button to the right, allowing you to cancel access right now. You can always come back to this at a later time if you modify devices or no longer want to use a specific website/service. This is the end of the security checkup if you are not using two-step authentication.
If you are using two-step authentication on your account, you may notice that some devices do not support the authentication code. For these, you would have used an application-general password. This section allows you to revoke any of the devices you no longer want on your account.
The ultimate section of the security checkup includes your 2-step confirmation settings, which should include a backup phone number and backup codes. The phone number will help you work out access to your account with Google if you lose other techniques of logging in, and the backup codes serve as one-use passwords if you lose the authenticator.
After finishing, you'll see a "Nicely done" page load, and the 2 GB of Drive space will be awarded to your account around February 28, 2015, according to Google. So hurry, and grab your free gigs now!