I'd check out the FAQ - which will answer many of your questions.
One important pointer is to post in the proper places (again the FAQ can assist you in figuring that out). If it's a costume, this thread is perfect. If you're looking to buy or sell something that's exclusively for the Junkyard... etc.
If you have any specific questions about the forum or a post feel free to drop one of the admins a PM - they are very helpful!