Building a bulletin board or a forum is not an easy task that you can build by your self, so what you should do is to use a ready bulletin board or forum application and install it on your web host, which is very easy. There many of these applications on the internet and most of them are for free, here are the most popular ones and how they can be installed:

phpBB
A Bulletin Board System, phpBB lets you build a website where users can leave public or private messages, for example, to advertise things to buy or sell, announce events, or provide information. most of the time the BB's are talking about the same subject. phpBB is one of the most used bulletin boards on the internet, because its absolutely free and don't even need registration, and its loaded with features and security is a priority for phpbb developers. phpbb supports many different database servers like MySql, PostgreSQL, Microsoft SQL Server, Microsoft Access