In order to upload your existing site in iWeb '09 (v3):
Click on the name of the site in the sidebar: the publishing settings pane will open. Set 'Publish to' to 'FTP'. Enter the name of the site and a contact address (if desired).
In the 'FTP Server Settings' section you will need to know the server address (your hosting service can tell you that), and your username and password for that service. Your site will be published in a folder with its name at root level of the server, with an index.html file at root level (which will overwrite any index.html file which may be there already). The 'Directory/Path' field may need to include a path such as '/webspace/' or 'ht_docs/' - this is dependent on your hosting service and they should tell you this. If you want to publish within a folder you can add that to the path.
You can then click the 'Test connection' button so that iWeb can check that it can get access to your server space. You should enter the URL of the site in the 'URL' field so that rss feeds and internal links have the correct address.