Lo and behold is used to draw attention to something important or startling. It isn't used as much in contemporary English, most of the time today it is used in a sarcastic or condescending manner. Here is an example: "Lo and behold, Gary decided to turn up to class today!" In this case the person speaking is being condescending to Gary by acting like his presence is out of the ordinary. If you wish to use it, begin your sentence with the phrase "Lo and behold" and follow it with the observation you want to make, but be careful because you might sound rude using it.