Hagia Sophia is opposite the Sultan Ahmet Mosque ( Blue Mosque ). Hagia Sophia is used firstly Byzantine Cathedral then Mosque and serves as a Museum today. The most glorious architectural heritage of Byzantine Empire is Ayasofya which has been refered to as the eighth wonder of the world and it is one of the most important surviving works Byzantine Period.
Google Maps - Hagia Sophia

Hagia Sophia was built under the auspices of Emperor Justinian I, who, along with Emperor Constantine, made great contributions to Istanbul. Massivve in size and reputation, the basilica stood for centuries as one of the world's most signifianc buildings. Firstly it served as the Patriarchal cathedral of Constantinople. The building was a mosque from 29 May 1453 until 1931, when it was secularized. It was opened as a museum on 1 February 1935. The mosaics of Hagia Sophia , which were uncovered after it became a museum, are the foremost examples of Byzantine art of the ninth to twelfth centuries. Similarly, Hagia Sophia also boasts fine examples of Ottoman tile and calligraphic arts.

How to go Hagia Sophia?
You can go Eminönü - Zeytinburnu Tram.
You get off Sultanahmet Station from Tram then you can reach to walk 1,2 minute to Hagia Sophia ( Ayasofya ).
You can reach from Eminönü or Beyazıt to walk.
İETT Public Buses - Which public buses to way Eminönü
İETT Public Buses - Which public buses to go Beyazıt