Some toughts to share:
By definition a community is commonly considered a social unit (a group of three or more people) who share something in common, such as norms, values, identity. Durable relations that extend beyond immediate genealogical ties also define a sense of community. People tend to define those social ties as important to their identity, practice, and roles in social institutions like family, home, work, government, society, or humanity, at large. Although communities are usually small , "community" may also refer to large group affiliations, such as national communities, international communities, and virtual communities.
By definition a forum is an online discussion group. Online services and bulletin board services (BBS's) provide a variety of forums, in which participants with common interests can exchange open messages.
By definition a topic is the general theme, message or idea expressed in a speech or written work. Effective writing requires people to remain on topic, without adding in a lot of extraneous information.
When people are interested in a community, a forum or a topic they make various activities related to it. Most of them make some research, trying to find readings and share it with the others. The interest can be driven by different factors like work, hobby or just something to be involved in. With various activities people gain knowlege and wanted to use it.
After it people try to use the knowlege in their every day life and work. With iterratvie usage of this knowlege people gain experience, mainly learning by mistakes. This will lead to brooden the worldview of people and they will gain additional knowledge and experience.
Share these knowlege and experience with the others by writing a blog post, reply to topic or just say something (by writing) in a free form. It doesn't matter how interesting it will be for the others, will they like or dislike it, the main thing is to be interesting for yourself.
That's it.