The culture of big city promotes the development of new types of art which decorate concrete jungles. Each megalopolis has its own unique character which can be best revealed through the art of urban photographer. There are no strict rules of what you should picture; it may be some building, city personage or some minor detail of urban landscape. It is essential to feel the pulse of the city and reflect it in your works.
You can get inspired by the works of the most famous urban photographers. Personally I like the photos of Keegan Gibbs who is most interested in shooting graffiti artists at work. Martha Cooper and her “Subway Art” book are also worth mentioning. Her works became a real discovery in the sphere of subway representation. I recommend also to have a look at the works of Norman Behrendt (his recent project is called “Burning down the House”), Jonas Lara, Edward Nightingale, who focuses mainly on train and subway graffiti, Ian Cox and Alex Fakso.