Perhaps you could be a little more expansive about what you're looking for? Budget? Interests?
Also, I would suggest you try doing a search. There are multiple threads in this forum about what to do in Istanbul.
After you've done that, perhaps you might then have a more specific question(s) that folks could help you with.
Without doing a search and just scrolling through the first two pages of this forum, I counted six threads relating to Istanbul and/or Turkey.