Problem with relative paths

I've been searching around here for a while and cannot see an answer to my problem, like a lot of others before me I apologise in advance if it has already been raised.

When I click the submit button the code tries to go to www.focusferrers.co.uk/addentry.php rather than
http://www.focusferrers.co.uk/guestbook/addentry.php

I actually want it to eventually point to http://www.focusferrers.co.uk/main.php?page=addentry but I know that wont work for the moment.

Where do I set the variable that makes it return to the correct page? I know I've put addentry.php in the root of my web page. That is so I could see what page it tried to go to. (It was just going to my ISP's 404 page.)
If there is anybody out there that can help it would be gratefully received.

should of said I start from http://www.focusferrers.co.uk/main.php?page=addentry