G'day and welcome to MacForums.
!st you need to set a gmail account up through google, then click the top left, then system preferences, then under Internet & Wireless click on Mail, Contacts and Calendars and you will be in a window with all types of Accounts on the right. Click on the Gmail one, put in your gmail credentials and apply and Mac Mail does the rest for you. Now goto Mail, and your new gmail will be there under whatever you called it in the setup.
Cheers