Since there are only so many hours in a day and only so much time you can devote to forums, newsgroups and mailing lists, look for a few that most closely match your market. Establish your presence in one group at a time and participate often. People with similar interests subscribe to mailing lists. When a member sends or answers an e-mail to the list, all members receive the message. Members also can communicate directly with other members.
Mailing lists have two broad categories, private or public, moderated or unmoderated.
Private list membership depends upon your fulfilling some requirement which can vary from professional qualifications to just giving your e-mail address. Moderated lists have a person who acts as a message gatekeeper. He or she will read all submitted messages and post only those meeting acceptability requirements. Thankfully, the moderator may edit rambling messages or produce a digest of messages. Without a moderator all messages are posted to all subscribers, who can end up with dozens of daily messages, some non-sensical, from the list.