Running a t-shirt business and running a forum are two completely separate jobs which each require a lot of time and effort to maintain. I'm sure Rodney can put in 2 cents there.
I have never seen a
successful forum run by a company for its customers. If someone out there has an example for me, I'd love to see it. All the ones I have seen are ghost towns or on their way to becoming one.
Most forums are focused on a very narrow topic and not just "general fun". Think about what your topic is - most of what you described is already covered by social networking sites like Facebook. Funny news/topics/pics websites are a dime a dozen.
I think the most successful forums are the ones that focus on one single niche and nothing else. Say, t-shirts. Or parenting. Or raising tiny poodles. You get the idea. If you can provide that, you can drag people away from social networking sites because your forum is unique and necessary. Otherwise, I'd say forget it, you will just waste your time, a few customers will sign up and be annoyed when you close the whole thing in a few months because no one else has joined.