Although you may not expect this, Berlin is a great place for shopping: The prices are usually lower than in other German or international cities.
Depending on what you want to buy, there are quite many areas to go to. For example, the Kurfürstendamm and the Tauentzienstrasse are regarded to be similar to Oxford Street in London. Here, you will find large department stores but also expensive boutiques. However, Kurfürstendamm will never be as crowded as Oxford Street. In addition, you should not miss the numerous side streets where there are many shops as well.
In Friedrichstrasse, there are also quite a few department stores and passages, e.g. the Friedrichstadtpassagen for rather exclusive products and designer clothes. Hackescher Markt also is a great address for buying designer clothes, particularly in small shops. In the rich Charlottenburg area, you find many posh shops and boutiques. For hip fashion, stroll around Prenzlauer Berg or the Scheunenviertel. If you are looking for second-hand clothes, head for Kreuzberg.
Even Alexanderplatz has become a good area for shopping within the course of times. Here, you find for example the huge Galeria Kaufhof, the shopping mall Alexa as well as many small shops in the surrounding area.
But as there is a huge variety of offers and possibilities to shop in Berlin, it is very hard to recommend a place you have to go to. You will probably find the best clothes and make the best bargains when you just walk around an area you like the flair of.
