You can choose to follow a post without leaving a comment; simply tap the bell icon under the post to start following. The bell will turn blue, and you will be notified of any future comments to that post. 'Liking' a post or a comment (by tapping the thumbs up icon), or writing a comment of your own both have the same effect: you will automatically start following the post.
To unfollow, simply tap the bell again. This will not remove your 'like' or your comment if you have made one, but you will not receive any further notifications about others' comments.
NB The bell was previously a star icon.