What sort of restaurant are you seeking?
Hamdi has good Turkish food and wonderful views if you get on to an upper floor (preferably the top floor with the glass roof). Address etc
here (it is down near Eminönü, where most of the ferries go from – when you're standing with your back to the wharves, it is up to your right, past the Yeni Camii (New Mosque).
There are a large number of restaurants in and around the Flower Arcade (Çiçek Pasajı), also known as the Cité de Péra, off Istiklal Caddesi (a principal pedestrianised shopping street that runs off Taksim Square). One is Boncuk, in Nevizade Sokak, behind the arcade, which does good food, especially seafood, from various parts of Turkey. However, as I say, it is one of very many in that particular location.
There are also some good places to eat in the Sultanahmet area, particularly in some of the smaller streets away from the big tourist attractions. For example, the
Sembol Meat House, despite its somewhat unprepossessing name, will do you a good Turkish meal in simple but friendly surroundings for a reasonable cost. But again, there are lots of others (although in this area I always avoid the ones that have an aggressive person outside trying to lure in the tourists).