There are so many spots in Berlin... The most popular ones are:
Kulturforum, and Opera at Potsdamer Platz
monument at Friedrichshein
and ledges at Warschauer Strasse (near S-bahn)
You can also check Skatehalle - skatehalle-berlin.de also near Warschauer Strasse.
At any of this spots you can ask for direction to go to ather spots, or the best way is to find someone who will show you around. To be hones I had a guide and just can't find on map most of the spots we were at.