Make a planning:

What does a forum need?
- Users
- Subforum
- Topic
- Posts
- BB Code

If you know this you can think of how to write it.
- Start with users (login + register)
- Create subforum list
- Create topic list under the subforum page
- Create posts under topic

These are the basics thoughts behind a forum. It's more easy than you would think. :)

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Nunchaku! Who doesn't like martial arts? =)