I understand where you're coming from. I came from a history of playing Freeform for a long time before making the jump to PbP. But Rena is right - break it down into manageable parts.
Outside of the ad thread, which is an important part of showcasing your prospective game to those who might be interested, every other thread can be made up as you see fit. However, for your reference, the last time I built up a campaign forum here, I set things up kinda like this:
*Main Folder - Announcements thread (sticky'd and locked), main RP thread(s), OOC thread
*Application Folder (for those applying for the game) - Creation Rules thread (stickied and locked), Creation Questions thread (stick'd), and everyone's character app threads
--sub folder: Completed Apps (move anyone's character thread into this folder once they've declared it complete, making picking characters a little easier)
*World Building Folder (optional) - as many threads as you need to flesh out the setting
The big thing here is the character creation rules, as that will answer a lot of questions that might come up. In the case of Pathfinder, for example, I recommend listing out starting level, class/race restrictions, starting gold, house rules (such as the Feat Tax Removal, or Background Skills), etc. If at all possible, take a look at another campaign's threads for reference.
Now as for your other questions, keep in mind that you will have your players as a source of inspiration, as well as any other inspirations you collect ahead of time. And I do recommend you find some sources of inspiration to continually reference. I use images I've collected to keep my mind in the right direction, as well as a lot of random notes. Also, it doesn't hurt to loosely and roughly outline your intended plot, but be ready to alter and adjust as needed.
Anyhow, that about sums up the stuff that ran through my head. Regardless, take your time and handle it all one step at a time.