Now that's a blast from the past, its been nearly 20 years ago I set up a BBS
About your question: Have you searched the net? Out of interest I did a quick search and many sites are out there that deal with BBS's and how to set one up (lot of windows stuff, but Unix/Linux entries are also present).
Be aware that some use older techniques (including BBS based on telnet) which might not be considered save in this day and age.