i will make one in flash
but is not something what can be explained in a board

i suggest this to follow this
1. select a server (linux, nt...)
2. select a database (mySQL, access, sqlserver, oracle, etc..)
3. server side language(cfm, php, asp)
4. the interface (flash, html)
5. start to work

or u can download some msg board ready to use like www.phpbb.com
runs on mysql and php... easy to install