As in any large, metropolitan city, Berlin provides a number of international standards in a range of neighborhood atmospheres. Although most cuisines have been adapted to some degree to German tastes, you can find Turkish, Italian, French, Cambodian, and much more on various avenues of the city; the kicker is that the cost will generally be lower than other European cities. You'll also find, despite the German reputation for meat-heavy meals, that there are some fine options for vegetarian dining.
When in Berlin, there are two “must try” dishes that can be found pretty much everywhere. The first is currywurst, which is basically a street-food staple of sausage covered in curry powder and ketchup; match it with crisp fries and you've got a typical meal. The best stop for this inexpensive meal is Curry 36 in Kreuzberg, although Mustafas (also in Kreuzberg) and Konnopke Imbiss in Prenzlauer Berg come highly recommended. The second dish is the Turkish döner, where lamb or veal is shaved from a massive spindle and wrapped in a flatbread called a durum with some other additives. There seem to be a ton of these shops in the city, based particularly around the Kreuzberg and Neukölln areas. Hasir Restaurant is the oldest in Berlin and has about five locations in different neighborhoods, while the hidden gem Tekbir Döner and the aforementioned Mustafas (which is always packed) are also fine options. Then again, if you're in these neighborhoods, there are a ton of kebab and other cheap dining options.
The most popular area of the city is Mitte, which has become the modern center since the dissolution of the Berlin Wall. Although you'll find heavy tourist thoroughfares, even in neighborhoods that had previously spoken of authentic, less-sterile Germany (Hakescher Markt, we're looking at you), there are still gems to be found for any taste. Most will want to stray from Oranienburger Straße as well, although cheap alternatives like the great Vietnamese of De Nhat still exist.
When in Berlin, because business turnover is inordinately high compared to other cities and hot new places are often popping up monthly, it may not be a bad idea to stroll around a neighborhood that is known for the style you're looking for. Other than the area's previously mentioned, Schöneberg in West Berlin is great for its cafe vibes and outdoor seating, while also providing a multicultural assortment of cuisines (Greek, Iranian, you name it); Friedrichshain, in East Central Berlin, caters to its large student population with pizza and kebabs, while great German restaurants (schnitzel!) like Spätzle & Knödel and Schneeweiss define authenticity.
For more information on cuisine in Berlin, take a look through our dedicated pages.