Welcome to WebForumz.

Don't know what 'sitebuilder' is but it sounds like a 3rd party building programme which may have only 'published' the files to a location on your own hard drive (if you know different, please disregard).

You need to upload the site to the webspace your host has provided you with.
It's usual to make the name of your homepage 'index', so the browser will find it automatically in the root file.

Make a note of the address of your hompage (something along the lines of [URL]http://YourHostingAccount/NewToThisStuffsRootFile/index.htm[/URL] [or 'html', as applicable]).
Then go to your user control panel and look for the domain name you purchased.
Where it asks you the address you would like it to point to, direct it to the address above (making sure you substitute all the rubbish I used for the [I]actual[/I] address of your index page) and VOILA!

Robert should now be your Mother's Brother.