In the left hand column of iWeb, select the name of your web site. It should be listed at the top, and indented below it are all your web pages. This should display the "Site Publishing Settings".
Ensure that "publish to" is set to FTP server, then look in the FTP server settings info area. There is a spot there for a password.
This password is provided to you by your FTP server. If you can't remember what this password should be, then you need to contact your FTP server.