Question about garage roof ventilation

I have a old detached garage around 25'x15', no insulation and has an open attic space. There are no soffet vents or roof vents and I was thinking about adding some to extend the life of my roof and preventing possible moisture buildup. Is this necessary? And if so what is my easiest but still effective solution? Im in the Midwest, garage is still cool in the summers and freezing in the winters.