One way to secure your Google Drive is through the two factors authentication. You can enable this feature using the following link: https://www.google.com/landing/2step/
Once you enable this option, every time you sign in to Google Drive, a PIN will be sent to your email.
Second option requires additional software. For example, Boxcryptor has this ability of encrypting the files stored on your computer. You can give it a try using the following URL: https://www.boxcryptor.com/en