Can anyone share how 1. Blogging and 2. Forums helps in a content base

I have been trying to get involved in couple of blogging and forum sites to promote my brand. I can write about niche market current topics and about the topics.

But, I want to know how good is this idea? and what is the best way to start this?

Most forum sites have restricted content topics. Under these circumstances, is it possible to promote any brand through various forums and blogging sites? How?

Would be grateful if you share your intel to prepare a basic content marketing strategy?