You can enter a post from the community home page. That's the page with your profile picture on the left and the Community menu below. Here's mine...
To enter a post, place the cursor in the empty box. Simply type what comes to mind, and hit the POST button on the lower right corner of the post box.
You can also add an emoji, a picture, or tag a friend.
To comment on a freind's post, simply put your cursor in the Write a comment box and hit return.