While iWeb is designed to publish to Apple's .mac hosting service it can be used to create a website that can then be uploaded via FTP. To do this:
Go to the File menu and select "Publish to a Folder"
When the Save dialog box opens, create a new site folder. This needs to be saved somewhere it can be easily found later (the desktop is usually a good place). After the Publish operation is complete, the destination folder will contain an index.html file and a directory with the name of the site.
Upload both the index.html file and the directory (with its contents intact) to your site via FTP
Please note: The directory you've published to should not be uploaded; only upload the index.html file and the subdirectory inside it. The subdirectory must be uploaded intact; do not simply upload the directory contents or the website will not function properly.
Tips
If you want to create an RSS feed for your site, use the URL box near the bottom of the Publish dialog; otherwise, leave the box empty.
If you experience FTP errors when publishing, the most likely cause is spaces in the file name. Check all files and all image names, then make any name changes in iWeb (Control-click or right-click the file name and choose Rename, then type in your corrections) and republish the folder.
Learn more about FTP.