Basic functions of this forum:
Home - takes you to the home page, obviously. This is wear you talk with others.
Calendar - I will sometimes post things there. Announcements, etc. Birthdays, if the users set that up, will be present there
FAQ - frequently asked questions. You may need to consult this before coming to me. I don't know everything technical about this site, but I will help you out as much as possible.
Search - If you want a certain thread or user, you can search here. Just put in the keyword you want to find.
Memberlist - This has all of the members on the forum. Click on the names to see stuff about the users.
Usergroups - Mostly used for houses. May have other groups. Idk yet.
Profile - Look through this. This is where you customize your account. Preferences, notification settings, etc. are all here.
Messages - If somebody sent you a Private Message this is where it will be located. If somebody is sending you rude message or whatever, let me know. "New PM" obviously creates a new message you can send to a chosen member. To reply to a a message, press "Post Reply." It's just like replying on the forum.

Definitions
Topics = "News," "Hogwarts," etc. Click on these to look through the forum. There are only some places you can make new ones in. You'll see "New Topic" button if you can. Sorry, I just don't want things where they shouldn't belong.
To reply to topics, there is a "Quick Reply" at the bottom of the page, assuming you can reply there. This is the easiest way to do it. There is also a "Post Reply" button that gives you a few more options.

I think this pretty much covers it.
My best advice is to click around and search out the forum. You'll figure it out eventually.