This is an interesting starter you have been given. The best thing to do with a question like this is to brainstorm or try to come up with as many ideas as possible to do with the prompt. What is the first thing that comes into your head when you think of the phrase "It only happens once"? Is it a particular story, event, character or action? Do you think of a particular time in your own life? The beauty about a task like this is that it is deliberately very open, so you can talk about anything you want to as long as you can link it to the prompt.
I will give you my ideas, but please don't be limited by them. Your speech will be better if you can talk about your own ideas. For me, the phrase summons up the many different times in our life when we have an opportunity that will never happen again. For example, you could relate this to going to college. You could therefore talk about going to college or something similar and how important it is to make the most of every experience, because "it only happens once" and you will not get a chance to repeat it. In the same way, you could also talk much more generally about life, as that happens only once, and we do not get a dress rehearsal! This fact should impact the way we live our lives. I hope this helps. Good luck and I am sure your speech will be very interesting!