how to read url at page init leve and rewrite it

I need to make dynamic website like google provide services of develop and design on line website,
I have completed all things of database and design, but how could i get the url at loading time
of the web application
if page not found then 404 error acre

in google

the website url would be www.website.google.com/ourlwebsitename
here ourlwebsitename is as a example of user created name.
same thing I want to do.

www.mywebsite.com/userchiceaname