Don't reinvent the wheel. There are many good forums out there. Some free some not. PHPBB
comes to mind as a forum that is highly customizable. If you try to recreate a full working forum it could take months because it requires that you know at least one server side language quite well, you will need to take of security issues. You will need to know how to work with a database. You will need to know how to allow uses to create a username and password for logging in.