Life-bar = Strife-bar

I really need to create lifebars. How do you create a lifebar? I already have my enemies taking damage, but I want to assign each of them a lifebar that appears over their head when you put your mouse over them. I don’t need you to help me figure out how to make it appear when the mouse rolls over it, but I do need to figure out how to make a lifebar. I don’t even know where to start. Thank you!