Newbie question: subdomain creation !!

[#1]

Hi ...

Hopefully an easy one. 

I have no problem creating a domain in the zone I selected (i.e., mydomain in zone ucsd.edu). 

How do I make a subdomain of mydomain.ucsd.edu?? (i.e., www.mydomain.ucsd.edu)

So far, I have an alias (www.mydomain) on mydomain.ucsd.edu. I'm betting that's not the best way ...

Thanks!