I toured Istanbul with Walking Adventures International, who engaged a licensed local guide. I highly recommend studying the history of Istanbul before you visit, so you can appreciate even more what you will experience.
I took along Rick Steves' Istanbul guidebook (Compare Prices), and many times our routes coincided with the walking tours in that book. We stayed at a hotel that was just a short walk from the Sultanahmet District.
What to wear: Cushioned walking shoes are important as the streets are made of hard cobblestone and if you arrive at a site the same time as tours from cruise ships arrive, you may be standing in line for long periods of time.
For touring mosques, both men and women should have their legs fully covered (no shorts), and have a shirt or jacket they can put on to cover their arms. Women should have a scarf to cover their hair and cleavage. You will also be removing your shoes when entering a mosque, so keep that in mind when choosing footwear and socks.
Istanbul has a continental climate - hot and humid in summer, cool and moderate in winter. Istanbul weather.
Istanbul is a relatively safe city for those who take basic precautions, and it is often packed with tourists from around the world. If you only speak English, you should still be able to navigate within the touristed areas. I found it to be a very clean, pleasant city to tour.