I forgot one tip: if you want to buy some pot, just go to Görlitzer Park and make eye contact with the guy very obviously walking toward you trying to make eye contact. They'll of course try to get you to buy more than you want, you just gotta insist on the amount/price you want. You'll get swamped by Africans trying to sell you drugs at Görlitzer Bahnhof, but it always seemed too shady for me, so I stuck to the Turks in the park.
Oh and in Neukölln, check out Syndikat. It's my favorite bar in Berlin. It's a proper collectively run antifaschist punk bar with a sweet atmosphere, cheap beer, and the best Mexikaners in Berlin. If you haven't had a Mexikaner before, go there and drink one. There's another punk bar called Clash Kreuzberg which is ok, but it's rather trendy and always packed with people who shouldn't be in a punk bar.
If you're into going out dancing, Berghain is the famous/infamous rave club in Berlin. I can't speak to the experience though. I never actually went clubbing in Berlin (a shame, I know) because I made the mistake of finding a girlfriend my first day I arrived in the city last summer.