You can just ride around on Bus #100 and get to see a lot of sights, but the bus doesn't offer commentary. The very best way to see Berlin is with a walking tour.
There are several companies that offer 4 hour walking tours for 12 euro p.p. Once you go on one and see how great they are, you will be hooked and want to go on more. They offer several different ones, a general Berlin tour, 3rd Reich, Cold War, some may offer a Jewish experience or Berlin underground, or side trips to Potsdam or Sachsenhausen Concentration Camp. Some get excellent reviews over on Trip Advisor, others not so good of reviews. Insider Tours, Original Berlin Walks, Fat Tire Bike Tours and if you want something more exciting a Segway tour, but that will cost you. The best part is being able to ask the guide all the questions you want, and get great tips for other places to go that meet your interest. Restaurants, events, museums, and so on.
Have fun, Berlin is great!