i want to check whether the user is exists or not in database...when user enters the user id into the textbox..if the user exist then allow successfull login and if not then redirect to new user...

So what is the problem did you find.on login button click pass the username as parameter to sql and writea query as select * from userTable where user name=@userName.
If user exist redirect to your home page and if not redirect to your user registration page.google it there is lots of example program availble.