Go to http://accounts.google.com/signin/usernamerecovery and enter any email accounts or phone numbers that you've had for a while and which may have been used as recovery info. The “find your email” page can help you locate any forgotten Google accounts.

After you select Forgot password and enter your username, we offer you recovery options in order to access your account. If you can't access these recovery options, you can click the link at the bottom of the page to verify your identity. You'll then be given a series of questions to verify that you own the account.

Tap the Gmail app icon on your home screen or in the App Drawer. Tap Sign in. It's at the bottom of the screen. If you aren't currently logged into your preferred Google Account on your Android, you can instead tap Add another email address, tap Google, and enter your email address and password when prompted.

To sign in to your Google Account (or any Google product): Go to the sign in page of the product (for Google Accounts it is myaccount.google.com). Enter your Gmail username (everything that appears before '@gmail.com'). Enter your password.