Continuing with what I learned last summer while visiting Istanbul:
Tom Brosnahan’s excellent site was my bible, or should I say Q’uran? I refer you to it with just a few tweaks/additional thoughts. Like I would add to his wonderful advice about stretching out a five-day museum pass into six days by starting in the afternoon at out-of-the-way Chora Church (AKA Kariye Museum): You might also want to see the Istanbul Fethiye Museum (AKA Pammakaristos Church) on the same afternoon, since it’s about a 10-minute walk from Chora and is another small museum that’s a little out-of-the-way but was very well-reviewed on Trip Advisor. And I suggest keeping in mind that a lot of places in Istanbul have more than one name for the same thing, like the two examples I gave above.
Be warned: Personally, I found it hard to find a lot of what I consider basic information, like where exactly (streets, intersections, et cetera) the tram stops are. Although I was staying in Sultanahmet, the only way I could find the Sultanahmet T1 tram stop was by walking along the tracks I spotted to the left of Haggia Sophia until I located the station up a hill I’d never gone before. And I think it’s important to know that whereas tram tokens are purchased at vending machines (Jetonmatiks, jetons being tokens), tokens are also acceptable on ferries but not on buses. For a bus you need a card bought at a kiosk selling things like candy and cigarettes. (There may be ways to avoid all this with some kind of transportation pass, but buying one was never worth it for me since I walk, even great distances, just about everywhere I go.)