As I mentioned when you attempted your last forum, the most difficult thing is getting people to join. You need a critical mass of membership before other members will join and it can easily be a catch-22 situation that you don't have enough members posting in the first place to make the site interesting enough to attract more members.
In my opinion you need two things:
1. A damn good idea for an original forum. I'd guess that most special interest forums already exist, so this is likely to be extremely difficult. Sites such as Facebook already cater for just general socialising.
2. Lots of friends or people who you can attract by whatever means to kick-start the forum into being active enough to attract members.
Personally I like the idea of a bilingual French / English forum. I'm English living in France but while I can speak and read French reasonably well I'm too lazy to join French forums where I would be compelled to write in French. However, there are lots of language learning sites out there such as Lang-8.
Why do you want to create a forum anyway?