The legal age of consent in Michigan is 16, subject to some important exceptions (like if the older person is in a position of authority or control over the younger person, like a teacher or a coach). However, speaking as a parent, my very humble advice to you is to wait until you are at least 18 -- you have your whole life ahead of you, and you may later find that this 19 year old is not someone you should have slept with. Either way, please make sure you use a condom every time to protect yourself. Warmest regards, Matt.
There is more than one way to get into trouble. Have you thought about how that is going to affect your life and his? This can all wait for you to get through school and get your life moving in the right direction in the adult world in a couple of years.