To get to the island, you have to take a ferry from either Mackinaw City or St. Ignace which is further north across the Mackinac Bridge. The ferries from Mackinaw City are more popular and, therefore, more crowded.
Shepler’s Ferry: At the suggestion of my hotel, I chose Shepler’s Ferry from Mackinaw City to the Island. Upon arrival at the dock, you can valet your car or use the self-park lot. They will then check your bag right to your hotel. This is so convenient! On the return, your hotel can check the bags with Shepler’s and when you arrive, it is easy to pick them up and retrieve your car from the nearby lot. Before you leave the island, if you have used the valet, they call ahead to make sure your car is there when you arrive. Just check the schedule as the frequency changes depending on the month and day of the week. They run from the end of April to the end of October. I would also book your ticket online as you can save a few dollars and wait in a shorter line upon arrival at the dock.
Star Line Mackinac Island Ferry is another ferry choice.