There are a lot of places in Istanbul to visit my friend.Let's start with Anatolian side...You may visit the islands first, there are 5 islands in marmara sea near the mainland;Kinali,Burgaz,Heybeli,Buyukada Islands..There is a monastery in Buyukada if i remember right :) and then you can wander around in Bagdat Street.It's nice and luxury place... I definitely suggest you to have trip on bosphorus :) You may love it ! and there are a lot of historical places like Topkapi Palace,Sultanahmet mosque etc..

Istanbul is a wonderful city its the cradle of civilisations

you should visit the east of Turkey and then istanbul.

istanbul of course and also ankara isa beautiful city ....maybe izmir antalya bursa

considering the diversity of the country, it really depends on what you're interested in. For example, historical sites, beach resorts, religious pilgrimages, etc.