For authentication on your own site, please start here:
http://msdn.microsoft.com/en-us/library/eeyk640h%28v=vs.100%29.aspx[
^].
This Code Project article could also be useful:
ASP.NET authentication and authorization[
^].
And after the user is authenticated, you would need to access some other site and authenticate on it. It depends on what this site requires (please see my comment to the question). Anyway, you can simulate any user's action if you use the class
System.Net.HttpWebRequest
:
http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.aspx[
^].
Most likely, that site has some log-on form. You can learn what's required if you learn this form (say, using your browser's "View page source"). On a form, HTTP request is composed of controls with the "name" attributes. But the general problem is close to the problem of
Web scraping:
http://en.wikipedia.org/wiki/Web_scraping[
^].
Please see my past answers:
get specific data from web page[
^],
How to get the data from another site[
^].
—SA