Using domain name with www

Hi,

I’ve started two wordpress sites on godaddy, using 2.9.1 and installed them in the root directory. On both sites when I type in the url like https://www.examplesite.com the website works but automatically transfers to https://examplesite.com

Is there a way to install this or change a setting so it will stay at just https://www.examplesite.com and is there a reason that I should stay away from this if it is possible.

Thanks for your help!