Better to start a blog first and then get the readers/subscribers. Then when your blog is successful and you are churning out some amazing content pieces that people actually find useful. Think about expanding that blog to a forum as well.
That way you've already got the reader base there in which you can tell them about your new forum.
Starting a forum from scratch in any niche is never going to be an easy task. Not impossible of course but not easy either.
First you have to think about what foruming software you want to use and you either go with one of the free ones like myphp or phpbb or something in which case you are pretty limited in what you can do and have to wait for them to release bug fixes / patches etc if any at all.
Where as if you go with the paid option you will have much more options available to you but that can be quite expensive and without knowing if your forum is going to be successful or not it could mean pouring money down the drain.
That said, its not to say you shouldn't go with either. Some people go with the free option at first, work hard at building the forum up with users/posts etc etc and then later make the move to a paid forum like Xenforo, VB etc. But that can be tricky and technical and expensive too! The drawback with going with a free forum software is that generally, the paid ones have much more features and people seem to like that today.
It all comes down to research. Finding a niche that is in high demand and finding out what most of those people searching it are looking for. Then there's the setting up part, you will need hosting, need security, need mods to moderate it. The list goes on and on.
Plus the make money online niche is super super saturated today! There are already many make money online forums out there. You need to ask yourself, what will your forum offer people that the other forums don't already? And why should people post to your forum with 5 members when they can post to other forums that have 500,000 members?
Just some food for thought for you.