You can fly into Detroit and then rent a car at the airport for an hour-and-a-half drive down I-94. Since you will have to drive through Detroit, make sure your flight doesn't get you there right before rush hour. Alternately, you can fly into Sarnia, ON (in Canada) and make the much shorter drive into the U.S. across the Blue Water Bridge to Port Huron.
There is no public transportation that I've seen. Rent a car or use a cab.
Fort Gratiot Lighthosue is viewed from the south end of Lighthouse Park. Take 10th Ave or Edison Parkway (riverfront) north to Garfield Ave. East on Garfield to Lighthouse park. You'll have to walk out onto the beach as the trees block the view from the parking area. It's also a great beach.