we started off as a graphics only site, didn't get it that far, so we have now revamped it to a brand new site for all round topics, Just go for whatever you think the public is interested in and you'll probably hit your match.
I'd go with whatever you enjoy doing, because if you don't like the niche of your forum then it will obviously die. If you're happy with a niche, you'll strive to keep it going and post quality knowledge about your expirences
I'd use a temporary domain since they are a bit pricey. I'd use www.dot.tk for starting out. But you should really pay for your host. NEVER use a free host. They are unsecure and vulnerable to any attacks.