Firstly you have to choose your heart desired niche for your forum and should be a forum that can be define and you must focus on that niche topic you can also put category like news,sport,business, webmaster e.t.c
You need a script to start your own forum and I will advice you to start with a free script and an example of such script is phpBB and it help web host offer as part of their package.
Now choose your own message board using a software that is if you are very a good web developer and then start creating you discussion area just as I mentioned above and make sure the are related to your forum topic.
You have to email or send messages to people you know either on Facebook or other platform after that you are good to start your forum discussion.
Publish post to you forum this will make people to know how serious your forum is.
Hope you will try creating one for yourself, if you can I think that is a great idea even as a blogger.
Don’t forget to drop a comment below thanks.