How about starting with making a login script? I usually start with the very basic, how I start (I'm the maker of a blog engine not yet released) is create a database with EVERYTHING you'll need. You'll want to get out paper and write down exactly how you want your forum to function (otherwise it could be a much larger problem when you run into small bumps later on) and then write out how the database needs setup. Then create and populate the database with some dummy entries (fake posts and comments, even before you have the functionality to post and comment, heck.. before you can do anything on the site) Then start pulling data from the database.
Once your site can successfully pull data and display it correctly from the database, start adding functionality for commenting and posting and such.
Don't expect anyone to give you an exact tutorial, this is a huge project, please take the time and do it one step at a time.