There are three things in the Plans Together forum: Subcategories, Topics and Replies.
Subcategories are containers for topics and other Subcategories. You don't need to worry about those much because they can only be created by the administrators of the system.
Topics sound like a big deal but are actually the main starting place for a discussion. A new question or idea starts its life on Plans Together as a topic.
Replies belong to topics. They are answers to or discussion about a topic.
This post, Posting Basics, is a topic. If you look below it and to the right, you will see a button that says Reply. This is used to make a comment about this explanation. If this was a question, that's where you'd put your answer.
If you want to make your own topic, in this category, you'd back up so you were looking at a list of topics, eg...
And there you see the New Topic button. Clicking it opens up an editor window in the bottom half of the screen. (If you are a person who wants to work on a bigger editor, click the black circle with an up-caret in it.)
If you have clicked on a topic and want to discuss it. Look to the lower right and you will see a blue button marked Reply. Clicking this will give you an editor, too. (If there are already replies, there will be more than one blue Reply button. These are there for convenience. It doesn't matter which you click. They do the same thing.)
You will see that the editor is very straightforward except for one thing. Unlike some online editors, this uses a preview to let you know how your post will look instead of trying to make the text you are typing on look like it will when published.
This is especially interesting when you make something BOLD. Clicking the 'B' in the control bar surrounds the selected text with asterisks. IE,...
That's because this editor is based on something called Markdown. It's a way of styling text using special characters. You can do a lot of things. If you click on the gray "compose" at the top right of the entry window, it will give you a link to a lot of information about Markdown that will help you make your posts look just the way you want.