Most forums take 1-2 years to write. What does that tell you?
Hmm.. if it takes you that long your trying to do to much.
Im creating one now, just gotta a few more templates to make, a bit of php to do, then start on the admin, its only taken 1month and thats pretty on and off.
But seriously forums arent hard, unless you try to have all this stuff to start off with..
If you want a basic forum.
So, maybe you want forum categories, than have forums in that category.
You may want a PM system and basically thats all a real basic forum needs besides creating threads and replying to them.
It can be quite easy, and if you want to add to it slowly you can do so, as long as you make sure you comment very well..