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.

On your computer, go to Gmail. Enter your Google Account email or phone number and password. If information is already filled in and you have to sign in to a different account, click Use another account. If you get a page that describes Gmail instead of the sign-in page, at the top right of the page, click Sign in.

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.