Before you can start a forum, you have to come up with a niche.
This is the main topic your forum will be about.
I'm a member of various webmaster or forum related forums. And I often see posts there from people that want ideas to create a forum about.
Of course it's a good idea to get some ideas.
But you have to keep in mind that those people probably know nothing about you …
They do not know what you like, what you dislike,