What you apply first depends on the kind of sunscreen you use. A physical block (containing titanium dioxide or zinc oxide) can be applied over your moisturizer. But a chemical sunscreen (avobenzone or oxybenzone), must penetrate whatever is already on your face in order to be effective. So its smart to apply this type before anything else.
In summer, unless your skin is very dry, you can probably use just one product: a moisturizing sunscreen.
Click here to learn more.