I am quite new on here and I expect this has been covered a million times before but how do I minimise the flies in my compost bin?

I started my first bin a few weeks ago and all is going well-I keep turning it with a fork etc and adding to it each day. All of a sudden, when I took the lid off today a load of flies were flying up in my face which was most unpleasant and i've been itching ever since!

What causes the flies and what prevents them?