There's a really huge amount of stuff to see.
The top sites are - Topkapı Palace, the old palace of the sultans. Aya Sofia, which is the old Byzantine Cathedral. Sultan Ahmet Mosque aka the Blue Mosque. The Basilica Cistern, which is an old Byzantine cistern, more interesting than it sounds. And there's the Grand Bazar, which some people find too touristy. Those are the top sites.
There are dozens of other sites, the Archaeological museum, a lot of other historical mosques, some other historical churches. There's Taksim which is the main square of the newer part of the city where all the protests are going on now and Istiklal Caddesi which is the main street in the new part of town which is actually from the 19th century I think.
Really there's so much to see, you need to get a guidebook or something. I'm not in Turkey at the moment I'm in Iraq but might go there next week, not to Istanbul though.