Like many cities you have virtually unlimited options for walking tours. We particularly loved Original Berlin Walks – Discover Berlin where you’ll get a fantastic overview led by a very knowledgeable guide (ours was getting a PhD in history) of the entire city in about 3.5 hours. On the walk you’ll learn about WWI, the 1920s, WWII, the Cold War, and modern era. Sites you’ll visit on the way include the Brandenburg Gate, the Berlin Wall, Hitler’s Bunker, Checkpoint Charlie, Nazi Air Ministry, Holocaust Memorial, the Reichstag (Parliament), Museum Island, the Bebelplatz (scene of the Nazi Bookburning), Topography of Terror, Neue Wache (War Memorial), among others. Basically you’ll hit all of the “must see” places.
Meeting point is outside of Starbucks opposite the Hackescher Markt S-Bahn station. Tours at 10:30 (though they used to offer more before Covid).
If you want a different tour experience, consider Brewer’s Berlin Tours (there’s a free tour at 10:30 daily), which is another great option. Meets at the square behind Friedrichstrasse Station (guide will be on the square between the Tranenpalast museum and Monki store).