Brooklyn11205

Go to the top of the forum you want to post in and you will see a box that says "start new topic". Click there.

If you want to join an already-existing topic, click "write a reply" at the top of the list, right side.

Actually I saw the "Write a reply" just before I wrote the note above. To comment in an existing topic, go to bottom of the screen and enter your note in the "typing box." (There's an internet-ish name for the box, but I'll never remember it.) After you type your message, click "post" just below it.