If you have an account with an Internet service such as MobileMe, Google, or Yahoo!, you can use the Mail, Contacts & Calendars pane of System Preferences as a central place to view, manage, and use your Internet accounts. You set up an account only once, in Mail, Contacts & Calendars preferences. After that, your user name and password is available to Mail, iCal, iChat, and any other applications you use with the account. There is no need to separately set up the account in the individual applications.
To open Mail, Contacts & Calendars preferences and set up your account, click Add Account.