Like I said, this is really a question that you should be able to at least start answering yourself. Think of it this way: Each table in a database stores a particular sort of data. What sorts of data does this forum have? Well, the obvious one is posts. Each post is a record in a posts table in the database. What else? Well, there's information stored about me and you, such as username, password, and profile information, so there's another sort of data that you need a table for.
Think about it a bit. Try to think of all of the different sorts of data you'll need to store and how is the best way to store it. If you can't at least get started on your own, then even if I did tell you exactly what tables you need a forum would still probably be too broad a project for you at this time.
