I have created a fairly big forum. Although im kinda upgrading it a little. maybe put a version to download for public for free in a few months or so. But when making a forum here are some words of advice
Write everything you want in the forum so
Members
count guests
posts
forums
admin
etc the list can go on for a little while.
Now from there brain storm each bit you listed. Say if its members only or both or guests only. Whats involved. What security measures you need.
From there
think of what you think should be recorded in a database. ie IP Addresses. Names, Username Passwords. etc etc.
From there you should then create a mini design. Work out each page what you want it to hopefully look like. Add a few minor adjustments to what you want in the site forums.
IF your happy. You should make a clean copy so you can understand,.
After that you should have whats needed in it. Database set up etc.
You should then make steps. So Make index page today etc. Rough finish date. Dont try and do too much. I try and finish a section or at least a file.
So maybe get the register bit done. That should include form, process and login. maybe even the usercp.
Now important thing. Its gonna take a very long time to do. Unless you got a team which can help debug each file. It could take maybe 3-4months depending on your coding abbilities. And how much you want in the forum.
Now Forums you must decide if your gonna use Cookies or Sessions my tip. Forums put a lot of stress on the server but although tampering cookies is quite easy. You should to use Cookies. Sessions just make it slower. Forums can be slow enough when they start getting real big.
The best forums are those that are Not too Big, But have features people would like to use. Example is vB although i think there are some features that are pretty useless.