Tourismus Service is next to the Hanover train station to the right on the next block as you exit the main entrance. You need to pre-book and pay for your tickets prior to the tour. Boarding is right outside the tourism office, on an island with two yellow mail boxes. A two-and-a-half-hour city bus tour of the main sights is €15 per person, every Saturday at 1330 (daily at 1330 from 17 March to 30 September).
We didn't find this tour very good for the English-speaking tourists, as most of the talk is conducted much more extensively in German. Only a light talk is given for the few English-speaking tourists. If you're fluent in German, you'd probably get better value out of it.
From journal ILM