In Ancient Rome, a forum was a public meeting space that was often used for political discussion and debate. In the present era, a forum is still a platform for public discussion; it's just the venue that has changed.
Instead of leaving the house to trek down to the town square, individuals can turn on their computer and engage in an online forum. With one click of a button, they can connect with like minded people all around the world. This is especially fortuitous for people with very esoteric interests, who may not know anyone in real life who shares them.
Take a look around before you sign up. Most forums allow non-members to read posts so you might want to lurk to get a feel of the community. Your fellow forum members might become your online friends, so you want to make sure you'll be a good fit. If you don't like the tone of their communications, you can always move on to the next.
Once you've chosen a forum, register your information. This is the part where you get to choose your user name, upload an avatar and edit your profile. Your details can be viewed by all the members so you shouldn't reveal any personal information.
Make sure you read the forum rules very carefully before you begin posting. Most forums have a 'don't bite the newbie' policy, but some can become very agitated if you break one of the rules.