Currently my office buy a third party certificate from endtrust, now i want to make the first page of the system will http, then after the user successfully log in it will https, how it can be done, please help me this is my first time to do this thing....

Please check this to see if it helps: Mixed Http and Https http://blogs.visigo.com/chriscoulson/mixed-http-and-https-content-with-sharepoint-2010/