good answer drlivingston, was tempted to give you a gold star

A little more information...you don't have to have a tour guide as with a guide book the city is fairly easy to explore...you could start in the old city Sultanhamet and visit the topkapi palace,blue mosque,aya sofya and the basillica cisterns.
There is a tour company that I've heard is reputable & professional but I've not had any dealings with them, called insight travel but if you do decide to use a guide make sure they are registered or have a license.
Have an enjoyable time in Istanbul.......regards jack