Multi site network user name and site creation

I have this question and don't know how handle this.

I setup a directory based multisite network and all I want is when user sign up instead of two options like registered user or registered a site on the sign form I want to setup in such a way on my signup form only one field of user name show up and in the back end Wordpress automatically use that user name and creat a website for him.
I hope I make a sense if not then in other way all I want is to just automatically turn user name into a blog name. Suppose is my site and customer is a user name I want when customer get registered his website is automatically created as
Any help would be appreciated.thanks