To log in, administer and maintain your G Suite (previously known as Google Apps) account, you would click on this link: https://www.google.com/a/YOURDOMAIN.COM
Once verified and setup, you will be able to access your mail on this link: http://mail.google.com/a/YOURDOMAIN.COM (Replace "YOURDOMAIN.COM" with the name of your domain.)
Here is a link to the G Suite Administrator Help Center should you require any further assistance.