Well generally if you have a new subject to talk about (i.e. a question about a species of fish, a type of filter, you have new pictures of a tank you've set up, etc.) you'll want to navigate to the appropriate section of the forum and click the "New Thread" button in the upper left. For example, say you have some questions about a certain species of catfish. On the main forum index page (http://www.tropicalfishkeeping.com
) you'd want to scroll down to the "Freshwater Fish and Aquariums" section. Under that, there's a big link that reads "Freshwater and Tropical Fish." (See the first picture). You can click on that to go to that subforum which has a general area for discussing freshwater fish as well as further subforums for the different groupings of fish (including catfish - See the second picture). Alternately, you could just click directly on the link that reads "Catfish" found under the larger "Freshwater and Tropical Fish" header on the main page of the site (see the third picture). Much of the site is only one link deep; for example clicking "Tropical Fish Diseases" on the main page takes you there and there are no further subforums within that area. When you start a new thread, you make a title for the thread (try to be both descriptive and brief). You also type up the first post that appears in the thread, which allows you to begin the discussion. When you're all set you click the button to submit the thread and it will appear as the most recent thread in that subforum, allowing other people to post in it. There are some other options when making a new thread, such as giving the thread a nifty icon or adding a poll to the thread, but the only requirements are a thread title and a first post.
It's generally bad forum etiquette to "hijack" someone else's thread. For example, if someone has a question about a goldfish and has a thread going about it, you should only reply to that topic if you want to talk about that particular topic. If you have an unrelated question, you should start a new thread about it in the appropriate section. To post a reply to an existing thread you can either click the "Add New Post" button at the top of any thread page or scroll to the bottom for the Quick Reply box. You can type a new post directly in that box and click "Post Quick Reply" or you can click "Go Advanced" to go to the same screen as clicking the "Add New Post" button at the top of the thread. This screen has more options for formatting your post and adding content to it (attachments, smilies, etc.) than the more basic quick reply does.
Other communication options include sending PM's (private messages) to other members. To do this, you can either go to your User Control Panel by clicking the "User CP" link in the extreme upper left of the page and then clicking on the "Private Messages" link on the menu on the left. PM's work much like email; you have an Inbox and a Sent box. You can also send someone a PM by clicking his username next to any post he's made and then clicking the "Send private message to _________" button in the menu that pops up when you click the name. If you view someone's profile, you can also leave them public messages (much like Facebook). You can also leave comments at other members' aquariums. On the left side of any post someone makes, under her user name, you'll see two tabs, User (which just displays the default info like the person's avatar, status, join date, location, etc.) and another that says "Aquariums." If you click that you'll get a list of the member's tanks, which you can click on to go to a new page with detailed info on the tank, including a box where you can leave comments. Finally, on the menu bar at the very top of the screen there's a button that says "Aquarium Chat." Clicking that takes you to a real-time chat room. The chat isn't always inhabited but sometimes people are in there and it's a good place to go for in-depth discussions or emergencies.
Hope that helps, and sorry for hijacking the thread!