On some boards you might be able to post and reply as a guest user. But most communities require registration.
A registered user can go to a forum on a board where he has permission to view Topics and leave replies. To reply you have a few options. You can click on the 'Post Reply' button and add a new Message to the end of the Topic. Alternatively, you can leave a quick reply in a Quick Reply box listed below the posts in the Topic.
If you want to Reply to a Message hit the 'Quote' button and you can Reply to a Message with the Quoted Text.