1. Set up the Coda Site (I assume you know how to do that).
2. Go into iWeb and Go to File->Publish to Folder. Choose the root folder of your site. iWeb will put the site in a folder in the root of the site.
3. Go into Coda and take everything out of the published folder located in the root, and put it into the root.
4. Upload it via FTP.
5. Done!
If you need to set up the FTP, then go into 1 and 1, create a new FTP user, and put the info that you get into Coda.
If you need more help, just ask.