www is fast becoming deprecated with people forgetting to add it. Some browsers add it and some don't.
How do i do this?
Basically you neeed to have
That way your server will hand over site were it requested with or without www.
Not sure where you can actually manage configuration on mac os x server. If you have default configuration (this site is (maybe only) default site). Then you don't have to do anything else than #1. In linux environment those config files can be found from /etc/apache2/sites-available. Mac prob have some control-panel for this.
I suggest taking a look at these similar questions.