Do you mean posting a new topic? You can do that from the homepage of the forums. On the right corner of the page, there’s a + sign with a button that says New Topic. On mobile, it’s just a + sign. If you click that, it’ll let you write the new topic. There’s a bunch of options from there, including a drop down menu that lets you select what category you’d like to post your topic.
You can also do it from each individual category. So, for instance, if I wanted to create a new topic in Meet, Greet, & Discuss, I’d go here.
If you mean replying to a pre-existing topic, then you have to scroll down to the bottom of the topic and hit the green reply button. I think you can also hit reply to the original post, and it should do the same basic thing.