How to start posting?… your doing that already, opening a comment box and writing out your views.

Knowing what board you should be posting in; you can virtually post in all the boards that a comment box can open into you. Once you’ve got the reply or new post icon on it, it’s most likely that you can post.
A couple of places you can’t post though, once it’s self regulated and you’ve been required not to make post on the thread or places where views from your rank isn’t needed.

As per knowing what board you should be directing your post to; all boards on the forum has got a title which defines the kind of subjects that is discussed within. You should be able to evaluate on best fit and in confusing situations, you choose the best fit and if it doesn’t fit, be cool if it gets moved.