The information that you put in the FTP server area, would be provided to you by your FTP server.
In the past, you published your web site to Mobile me....which is Apples server. Apple has discontinued this server, so you need to find a new server to publish to.
Many people use "Go Daddy", however there are many FTP servers out there. Do a google search for "iWeb friendly FTP servers" and you should get some ideas of other servers you can use.
You simply call one of these "server hosts" and tell them you need to publish a web site to their server, and they should be able to explain what you need to do. They will also provide you will all the information you need to fill in the FTP server settings in iWeb.
For information on Publishing your iWeb web site to an FTP server, check out the following link - http://www.apple.com/findouthow/web/#ftp. Also see - http://support.apple.com/kb/HT4686.