Summer in Prague is definitely beautiful, days are quite long and temperatures warm up considerably for July and August.
Summer is is from June to the middle of September. The summer season is normally mild (from 20-27°C) and rather humid, and the highest temperatures reach about 30-35°C for short periods, generally in July and August. This is the high season when most travellers head to the city, therefore don’t think you will be alone. Late July and early August can even be very hot and humid. That time also many locals head for their country cottages and the streets become a bit less crowded. Days are long, warm and the tourist attractions are open until late. It is lovely to stay out late in the open air, maybe having dinner or drinking in a Prague beer garden. Nevertheless, it is advisable to bring along an umbrella as rainy days are quite common in the summer, especially in June.
Consider bringing with you a light jacket, umbrella, hat, sunglasses and possibly sunscreen. And trousers – don’t pack only shorts!