My best advice:
Start a niche blog and when you start to get a consistent volume of comments, you know you have enough interest and traffic to possibly start a forum.
Activity breeds activity and a busy blog is FAR better than a fairly empty forum with some paid posts. Blogs are much easier to monetize, too. Forums are born from demand by consumers - not because admins want to create one. There are thousands of sports forums out there - what makes yours remarkable?
Don't mean to be harsh, but communities aren't created - they develop on their own and as the owner you respond to their growth by changing formats or adjusting the rules. I just hate seeing people put in work that will get no return, and forums are pretty much that kind of time suck (usually). The the same amount of effort into a niche blog and you are more likely to get somewhere.
Laura