You can reply to a post in the post stream or by selecting a single post.
In the post stream simply tap “Reply” at the bottom of an individual post and enter your reply in the text field. You can also add attachments to your replies.
If you tap the body of a post you will be taken to single post view and can enter your reply at the bottom of the screen. Single post view will allow you to see all of the other replies to a post as well.