I learned by taking other people's code and modifying it to do what I wanted. A good place to start might be to download any one of the million free bulletin boards that already exist and modifying it to do what you want.
If you start writing the bulletin board yourself, it's likely that one of two things are going to happen. Either you will get discouraged when, in four months, it still doesn't even let you post a message. Or, in four months, you will be much more skilled and you'll see some of the mistakes that you made early on that are now firmly embedded into your code. In my opinion, you will learn much more by starting with other people's code and examining how it works.
If you really want to build it yourself, here's where you start: Make a page with a form on it that posts to another page that writes the form's contents to a database. Then make another page that reads the data from the database and displays it on the screen. Make it so that each form post adds a new row to the table in the database and the page that displays the database contents displays all the rows in the database. We can add user accounts, passwords, and threads later.
