I really don't know anything about making an htaccess file but I do know that it has been talked about quite a bit here. Try clicking on "site search" at the top of this page and do a search for htaccess.
It sounds like you have wildcard subdomains enabled on your account. It might be easier for you to ask your host to turn off the wildcard option, and then make up a custom 404 page with a link to your main page from there.
Marcia: Wildcard means that *.domain.com will go to one IP. So www.domain.com, domain.com, and whateveryouwant.domain.com will all point to the same IP. A server still could slice and dice the requests to different sites with name based hosting. I do this a lot.