Thought I'd just offer a few in-between steps in case you need some help
To publish your site to a folder, ready to be placed on your ISP web space, go to
File >
Publish to a folder... in iWeb. Choose a folder then click
Choose.
Once you have installed one of the FTP applications mentioned in earlier posts, you will need your ISP's FTP address in order to connect to the appropiate folder. It's usually something like
users.ispname.com or
ftp.ispname.com or something. Your ISP should have this info on their website somewhere.
Once you have the info, use the server address, your username, and your password to connect using the FTP app. When connected, you will see a folder structure like any other on your Mac.
Upload all the files in the folder you published the iWeb project to.
Once they are all uploaded, anyone can access your site using the appropriate address. This is different from ISP to ISP. Usually it is something like:
http://users.ispname.com/~username/website.html
post back here if you get stuck