Spirit, citrus, and sugar
A sour cocktail is made of spririt, citrus, and sugar. An egg white is sometimes added.
I've seen 2:1:1 suggested as a formula: 2 parts spirit, 1 part citrus, and 1 part sugar. But I dont find this to be work often enough to recommend it.
Spirit, citrus, and liqueur
Spirit, citrus, sugar, and seltzer
Simple summer cocktail, use any fruit
You probably have everything you need in your kitchen right now