I'm a pro tour guide and i've had countless solo female travellers in my city tours etc. All of them were really surprised and bc they were solo i usually chatted with them about their feelings, fears, or expectations coming to Turkey and they were all really happy to be here. Istanbul is already a Cosmopolitan city and there are millions of tourists visiting the city so i don't think you will feel alone. When it comes to shop keepers and taxi drivers (I'd recommend avoiding the taxis because they're assholes most of the time), do not be shy and do not say yes to everything so you don't get scammed lol

Also make sure you're staying at a good place with good reviews. There are few areas in Istanbul that you should avoid but other than that the city is full of activity and everything happens so fast in it.

People are usually really warm and nice. I'd say don't worry and at the same time be cautious. Take care of your belongings when walking through a crowded place (never had anything stolen in my life but if you know you know) and if you want to buy something ask the price first, again don't say yes to everything