Just like any business. You need to develop a business case.
You need to decide what niche you want to fill and how you will go about filling that niche.
I don't think you really need to be a expert per say in the beginning. But it should be something that you have a interest in.
Otherwise you will just get bored to death and not invest the time needed to promote your site.
And who says it has to be a forum. It could be anything.
