Surprisingly little, mate.
1. You need a domain and hosting. Easily had for US$10/month, though you might have to pay the first year up front, so figure US$125-ish.
2. You need forum software. PhbBB is free, as is Simple Machines (aka: SMF). Both offer themes and plugins for additional functionality. VBulletin is pretty much the standard these days, but it will set you back something like US$300 for it and that only entitles you to updates for a fixed period of time. They've got a monthly-hosted plan, though, which might be worth considering. Not sure what software is used here, but I've seen it around.
That's pretty much the nuts and bolts of it, but then you need to figure out your index - main forums, sub-forums - then figure out how you want to handle user groups - newbies, moderators, admins, etc - and so on.
Personally, I prefer SMF. Hope this helps.
Brian Driggs | KG7KCA | PHX, AZ | 89 Pajero
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