Sightseeing:

It takes time to see Istanbul properly, since examples of its 2,000-year-old past can be difficult to find amid the chaos of the present. Most visitors go straight to the Sultanahmet district, the oldest part of the city, where there are at least some remnants of Byzantine Constantinople, such as Hagia Sophia, among the splendors of Imperial Ottoman architecture, such as Topkapi Palace and the Blue Mosque. Sultanahmet is, however, a ‘tourist quarter' with the usual irritations of badgering touts and crowds of people and it is not the only historic part of the city.

You can participate in daily city tours organized by Gemini Tourism. For further information please download:
GeminiDailyTours.ppt

For more information:
http://www.worldtravelguide.net/city/58/sightseeing/Europe/Istanbul.html
http://english.istanbul.com/Guide/GuideSubLinkAll.aspx?CatId=37
http://www.lonelyplanet.com/turkey/istanbul/sights