What version of iWeb are you using? If you are using iWeb 09, then open iWeb and select the site name of the site that you wish to publish and then click on the drop down menu at the site with publishing options and select where you want to publish - either a local folder or ftp.
If using iWeb 08, then your only option now is to publish to a local folder and then use Cyberduck to upload your site to your web host - you need to register a domain name and also purchase hosting to be able to host your site. If you don't have either of these, then take a look at http://www.godaddy.com and http://www.hostexcellence.com, companies which provide both.
iWeb offers two methods of publishing a website....
... and there are plenty of choices for hosting...