We were in Turkey in March, though in the middle of the month. It may rain in Istanbul at this time.
During the day it was already very warm, even hot, many even sunbathed – summer clothes, light trousers, thin jeans, T-shirts, short-sleeved blouses and more are quite suitable.
And in the evenings it is cool, it will be enough to wear a sweater or a windbreaker, a light jacket. Sneakers, moccasins and other demi-season shoes are more comfortable on your feet. During the day it can be +25, in the evening +15+18.
You should not take a lot of things with you, there will be a great opportunity to arrange shopping for yourself, in Istanbul there are many markets and shops with good quality fashionable clothes at low prices.