You can choose to follow a post without leaving a comment; simply tap the star icon under the post to start following. The star 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 star 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.