much easier to adapt a premade one to your needs...there are several very good low-cost/no-cost ones out there...first you'll need to know what your host supports...ASP or PHP...and what database they support/offer in your hosting package.
If your host offers ASP/Access/MSSql for your web you can look at www.soopportal.org (NOT .com) That is the one I use...great bunch of guys, its totally free, and good support..and lots of skins & mods for it.
If you're on unix/linux you'd need PHP/MySql there phpbb.com (i think thats the url.
Snitz is another one that comes to mind.
Your host may even offer one. My host offered Snitz installed for only 50 bucks.
You should install it into a subweb so FP will not mess with anything. Again, check with your host to make sure they allow subwebs.