Depends what kind of budget your on! If your looking for hostel style I stayed at the Circus Hostel last October when I was there. 4 bed mix dorm was €21, it was an awesome hostel, had a great tourism desk and info, the staff was helpful, the underground was literally across the street, you were only 2 stops away from Alexandraplatz. I definitely recommend doing a free walking tour on one of your first day, great way to meet people and you can get your bearings of the city, it's huge! A day trip to Sachsenhausen concentration camp just north of the city, it's only a half day trip so you can still do stuff during the later part of the day. My favourite museum by far was the Jewish museum, the DDR was okay, I found it to be very busy and overrated. I loved the east side gallery and just walking around Alexandraplatz. My favourite place for a night out was Friedrichshain and Kreuzberg. Very accessable from the hostel and only a €5 cab drive.
I spent roughly €50-€55 a day, that was including hostel accommodations, eating out for every meal, a couple nice sit down ones, binge drinking every night and all my attractions and public transportation, including for cab rides.
I spent 10 days in Berlin and could of easily spent more, it's my favourite city in the world! Any questions you have ask away and I'll do what I can to answer them!