What is a subdomain? Subdomain is a part of the main domain. For example: the address of the blog is: http://movie.thementalclub.com. Here, thementalclub.com is a main domain and movie.thementalclub.com is a sub domain. I will show you that how do you make a sub domain through cPanel. For making sub domain, you have to log into cPanel. Now, you click on the sub domains which is inside the domains.
Like this picture, you give your choice able sub domain in the box of sub domains. In Document Root, if you browse in sub domain, then you will see the information and you define the location of the information.
Now, if you click on the create button, then your sub domain will make. Now, if anybody browse in the sub domain, then for this sub domain anybody will watch the information in Document Root.