How Login To Another Web Site With Redirection from my asp.net wb site?

To log into another website from your website..you need to have the follwing things:

1. A common id to link between both the databases of different websites(Assuming both sites are using different databases)
2. A landing page on 2nd site where the page will redirect to from 1st site.
3. On the page from where you are redirecting, send the common ID to landing page of 2nd site.
4. On the landing page, get the common ID and validate against its own DB and do the login.

Another solution:
Use 3rd party logins such as OpenID and if the user has registered on both sites with same OpenID, follow step 3 and 4 mentioned above just by sending openID of the user and you can skip form step 1 and step 2.