Don't waste your time, go to Old Town Trolleys instead
Try getting a tour, good luck. For the money they charge, you would think they would have seating instead of having to reserve DAYS in advance. They offer online reservations, but those are limited. Plus they charge you $3.50 for the "convenience."
Instead, try going to Old Town Trolley tours. You can pick them up in several spots. We got our tickets at the Sheraton hotel (outside) in Back Bay. The trolley driver was the best, and you get a great tour where you can actually see, not be crammed in on a bench like the Duck Tours.