Google Calendars (here) are provided with every Gmail address, Google Sites, or other Google account. To see if you already have a Google Calendar, you can log in by clicking here or opening your Gmail and clicking on "Calendar" in the upper left (to the right of Gmail, left of Documents): 

Or simply type click on this address or type it into your search bar:  www.google.com/calendar 

If you aren't already set up, follow the prompts.