Make some fake accounts and post topics with them. When there are enough ppl, delete the fake accounts.
Ppl will see that there IS any activity going on your forum, so they might register and post some things too! ;)
For a domain you you'll have to pay most off the time.
For webHOST I'm not really sure. I know that with Apache you can run your website on your own PC, but I don't know how to make it accesable for others.