Website login

After clicking on login button, this will be opened in browser, right?. Not a problem read the 'Yahoo developer network guide' to develop such application. point here
http://developer.yahoo.com/dotnet/
http://developer.yahoo.com/dotnet/howto-bbauth_cs.html

Thanks Kumar bro, But my problem is little bit different. I am going to give u an example.  
                                I want to login to yahoo. So what my project do is to make a windows form in c-sharp. It will get input(Username, Password) and user will press the login button. Under login button i wil have to code for opening yahoo login page. and then what my application do it automatically insert the username and password into the textbox of yahoo page which were entered into the windows form by user and it execute the login button code so that my account login successfully to yahoo mail.
                                Hope you will understand my problem.
                                                                                   Thanks
                                                                                              Zeb-ur-Rehman

read this article

http://www.c-sharpcorner.com/UploadFile/abhikumarvatsa/1686/