Posting a thread here is actually quite easy.
The first step is you want to visit the list of forums, here
Then pick the most appropriate category, and click the "New Thread" icon, its silver and has a purple fish on it.
Then on that page that comes up, write your title and write the body of the post.
There are additional controls, its like a word processor and has all types of formatting and image embedding options, attachment options, etc.
Very similar to writing an email.
Then click to "Preview" your thread. If all looks good, YOU MUST THEN CLICK POST. If you "x" out of the page before clicking post, all you did was preview what you could have (and should have) actually posted.
So, first you want to preview your thread, then you want to make sure you post it.
When you do successfully post the thread, you will get a success message that your new thread has been posted. Immediately it will then be viewable to the other members who are either:
searching for new threads since their list visit, or
searching the list of that day's threads, or
viewing the homepage (it will appear there in the list of recent threads also, to give it good exposure), or
it will also be viewable to people viewing the list of threads in your specific category that you picked to post it in.
In addition, the thread will be syndicated out to other places to make sure people read it, such as Twitter, Facebook, Google, Yahoo, Bing, and various other places that are listening for certain keywords. So for that reason, when you post your thread, make sure to use a lot of descriptive keywords so it appears in front of people interested in the topic, so it gets picked up by several sources.
Well I hope the info helps and gives some additional useful pointers. If you have any more questions about posting, please let me know.
We also have videos about how to insert pictures in your threads:
Posting Pictures Tutorial - AquariumForum.com
The more you post the easier and more familiar it will become, but don't be shy or intimidated, its rather easy to post a thread and if its not, I'm always happy to show you how and explain farther, so just let me know.