Delhiites complain about many things in their city, but the food will satisfy even the most demanding gourmet. Not only can you find some of the best Indian food on the subcontinent, there is also an increasing number of excellent (if often pricey) international restaurants offering cuisine from around the world. When ordering, do remember that Delhi is about 1,000 km from the nearest ocean, so vegetarian, chicken and mutton dishes are the way to go.
Delhi has arguably the best street food in India. However, do not eat unhygenic or open food. There are plenty of restaurants offering street food in a potentially more hygenic environment (but still the best taste is found in the streets). Enjoy the street foods but keep some tropical medicines for GIT problems (Norfloxacin Tinidazole composition works very well)
You can join local groups of foodies who go out regularly to sample and savor what new and old dishes the city has to offer. One of the most active groups is Food Enthusiasts of Delhi. They organize regular food walks, better known as Raids to various parts and joints in the city. Its a non-commercial group, brought together by passion and love for food. If you are looking for professionally run setups, Delhi Food Adventure runs commercial food walks exclusively for tourists. If walking around looking for good food is not your thing, have a look at some of the Delhi-centric food and eating out blogs, such as Dilli Daawat.