My b2evolution Version: 2.x

Hi, I just installed b2evolution in to my root directory. when I put in my URL - http://www.myurl.com - I get to my b2evolution page but every link I click withing b2evolution goes to a URL without the www present.

For example the link to "home" on the home page is http://myurl.com/ NOT http://WWW.myurl.com

I want to run the site with www present all the time

can someone please let me know what I need to change to do this?

thanks