To access Gmail, you must first create a Google account. Go to Google's account creation page, fill out the required information, agree to the terms and policies and then verify the account when asked. You can then proceed to use Gmail services.
The username and password used to create a Google account are the same credentials needed to login to Gmail. Remember to pick a password that's hard to guess and enable the two-step verification for enhanced security. Though optional, one can associate their newly created Gmail account with a phone number or with another e-mail address for recovery purposes.