Are you going on the Best of Istanbul tour or the Best of Turkey tour? If the latter, then you'll need a variety of things, including the conservative wear and a head scarf, as mentioned above, especially for entry to mosques.
For some of the sightseeing days at places like Aspendos, Pammukale and Ephesus, you'll want sturdy shoes to scramble over uneven stones, stairs and paths. In October, it's likely you'll still want a sunhat/sunscreen. If you're taking the optional boat ride in Antalya, you may wish to bring a swimsuit and coverup as we were allowed to swim and dive off the side of the boat.
I took both tours last May and you can find more details about clothing in my Trip Report:
And I've answered a lot of the questions that I've been asked about the tours in this post: