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 the next page, you'll be asked for your name.

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.

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.