If I understand you correctly, you want a simple password protected page to display a webform. Then you want your visitor to upload a file to a specific location on your forum. Am I correct?
If I'm correct then this is pretty simple.
For the password protected page you can do two things, Protect the directory with .htaccess for that you simple place the page you want to protect in the protected directory.
HOWEVER, if you want to protect individual pages, then the best way that I know is SESSIONS and MySQL database.