How can l use the DotnetNuke to make a website like this http://www.doctorspring.com/
IS a site where people can chart with a doctor

There are several chat tools available, including the Olark thing that we use here, and dnnChat, a module that's more of a chat room than one on one chatting. You will find other modules if you search for "chat" at store.dnnsoftware.com.