Login System with access levels

Hi, I've got a db with a username and password in it. I can let people log in, like SELECT * FROM table WHERE username = username AND password = password. But how can I add an access level column so that I can have different levels of security. So admin's can read everything, but users can only read certain sections. How could I add to my db and structure a query? Any ideas would be good, Cheers, Jord