3. Spend an evening setting up and configuring the forum.
And the rest of your life nursing the bloody thing into continuous running

In all seriousness, the first thing to do is check is that there is not already a suitable forum already out there for the subject you wish to cover. No point setting up a new forum if there is already somethign suitable.
Once you decide that you do need to start one, then the next step depends on your IT skills. If you want to stick with Windows, something like Community Server works well, and is initially free for non profit organisations. More choice with Linux based forum software, but a few extra complications. Many shared hosting companies do not like forum software due to the resources it uses.
phpbb in particular needs constant fixes applied due to flaws, but remember that it does need security patches applied - you are holding people's personal details (such as email addresses), you have a responsibility to do all you can to keep it as secure as possible.