How can i start a unity forum?

What do you mean by "start a Unity forum"? So you want to create a thread on the forums? If yes, there should be a "Post New thread" button in black with white font when you are in one of the sub forums. It's possible that you need do post a few times in other people's threads before you can create your own one (probably an anti-bot procedure).

Uhm, sorry, but, what?

I really don't get what you're asking here. There are basically two websites: the UnityForum and UnityAnswers.

The UnityForum is an internet forum where user can post "messages" usually grouped in "threads" / topics.

UnityAnswers is a Q&A site which purpose is to allow user to ask questions about game development with the Unity engine and have other users answer that questions.

"Starting a forum" would mean that you want to host a similar site as the UnityForum. I don't think that's what you want to do, do you?

If you wanted to ask how to start a thread / topic on the UnityForum that would make more sense. The Unity Forum is split into several categories. You can only create new threads / topics in the appropriate subforum. $$anonymous$$ake sure you read the sticky posts at the top of each forum, so you know the rules and what topics belong to that forum.

sharing on the already existing forum and answers is more helpful than splitting knowledge up.