Go to https://support.google.com/accounts/answer/27441 to begin.
A Google Account can be enabled for an existing email address you already have, or you can create a new Gmail address and let Google host an new email account if you want to.
If you want to use an existing email address, be sure to follow the instructions for Use an existing email address.
When possible, RK9 Labs recommends that you use the same email address for your Deck List Creator Google Account that you use when registering for tournaments. This will prove your identity to both systems, and is the easiest way to access your registrations to submit a deck list to a tournament. (If the email addresses don't match, you will still be able to submit deck lists using a longer procedure.)