Wonderful! It's an amazing city you'll have a fabulous time!
I went about 4 years ago and stayed in Sultanahmet - it's the perfect part of the city to stay to see the likes of the Blue Mosque (amazing, especially lit up by night!), Hagia Sofia, Topkapi Palace (you'll need a full day to see all of Topkapi!), the basilica cistern and there's a brilliant tram service running through Sultanahmet so you can easily get to the Grand Bazaar, Spice Bazaar, the Bospherous and other places which lie just outside of Sultanahmet.
I did just one organised tour when I was there which was a day cruising the Bospherous, followed by a visit to Pierre Loti cafe which has fantastic views over the Bospherous (just a shame it rained that day!)
Make sure you buy a decent guide book and try and plan what you're going to see before you go... it's a huge place (yes JT, more like a country than a city!), with such a vast mix of people, wonderful smells, noise and bustle - it's an experience and a complete contrast to the sleepy summer resorts!
As with any other big city keep your wits about you and you'll be fine.
As I said it's the most amazing place, you'll get neck ache from looking upwards as everythings so high! I'd love to go back again, I spent a week there which just wasn't long enough!
Mazz