ASP.NET URL redirection

What you are looking for is called Forms Authentication. A very short introduction follows.

You need to create a login page that makes a call like this, after verifying the identity of the user:

FormsAuthentication.RedirectFromLoginPage(userName);

Then you need to wire up the login page in the web.config file:

<system.web>
  <authentication mode="Forms">
    <forms loginUrl="~/Something/Login.aspx" />
  </authentication>
</system.web>

Furthermore, you will need to tell the framework that all URLs below ~/Admin/ requires the user to be authenticaticated. This can be done by adding an another web.config file within that folder:

<system.web>
  <authorization>
    <deny users="?" />
    <allow users="*" />
  </authorization>
</system.web>

Read the article linked above, or search the web for "ASP.NET forms authentication" and you will soon be on the right track.

EDIT 1 - If all you want to do is really to "make a redirect to a specific URL", then this is sufficient:

Response.Redirect("~/Something/Login.aspx")

From the URLs you mention in the your questions, it seems that you are trying to enforce some kind of authentication/authorization scheme. If this is true, forms authentication is a better answer.

EDIT 2 - If you want to rewrite, not redirect, requests from ~/Admin to ~/Something/Login.aspx you can do so by mapping a URL mapping in your root web.config file

<system.web>
    <urlMappings>
        <add url="~/Admin/Default.aspx" mappedUrl="~/Something/Login.aspx"/>
    </urlMappings>
</system.web>

In most setups, the web server will only pass the request to ASP.NET if the requested URL ends with a known suffix, such as .aspx. On approach to trick the web server to pass requests for ~/Admin to ASP.NET, is to use the "default document" feature in the web server. For this to work, you must add an empty file named Default.aspx in the ~/Admin folder.