You surely took part in a forum or two where you found that it is also another way to earn extra money. Then why should you not initiate your own forum? Choose a topic of your like and make a forum based on it where people can discuss about it and share their ideas and experience concerned with the topic with each others.
When your forum becomes popular then the people joining you, you may charge from them or set a special room in the forum where one has to subscribe who take part with it. An example of that would be the WarRoom which only paid users of worrier forum have access.