how to prevent an user to type URL of my web site before signup

It is not possible.

As Mudassar said ... prevent somebody to type any URL is not possible ... But still you can achieve your goal via many techniques:

1- Via saving credentials in DB, then website confirms such credentials in order to open.

2- Via Request.QueryString, where SignUp process is hardcoded to produce some QueryString then Response.Redirect to the website. Website must confirm for proper QueryString before opening.

3- Via Session Variables, where SignUp process is hardcoded to produce some Session Variables then Response.Redirect to website. Website must confirm for existence of the Session Variables before opening.