At the very top of your screen, there is a tab called "user manual". This should answer most of your questions. There are specific rules for the Sale or Trade forum. These are posted in a sticky thread near the top of that page called "Read before posting!"
I've had a look at the user manual, but it doesn't tell me what I need to know.
Specifically, I've just finished setting up a planted tank directory, and I'd like to post about it on the forum. As well as letting people know about it, I also want to ask for feedback - comments, suggestions etc.
Would this be OK? It's a non-commercial directory, no paid listings or anything.
If you like, I can PM you a link so you can have a look.