Making a forum that (1) works, (2) is reasonably easy for the forum members to use, (3) has sufficient administrator/moderator functionality, and (4) is not a gaping security hole into your web site/host is very much a non-trivial programming job. That is why sites like this use pre-existing 3rd-party forum applications instead of reinventing the wheel and building their own.
And that is why I would suggest that if fairly basic things like setting up a user account database and the functions to interface with it are currently daunting to you, it would be much quicker (and safer) to use one of the many packages already out there. To get an idea of what is available you might want to view this thread at WebDeveloper.com.