The Patonus Charm is incredibly difficult, and usually is not learned until you have lots of wizarding experience. The way to cast the Patronus Charm is to think of your happiest memory, the single, solitary, happiest moment of your life. Then channel that all into your wand and say, "Expecto Patronum!" If done correctly, your favorite animal should burst out of the end of your wand and scare away the dementors. I recommend trying this on a target first, rather than going out and finding a real dementor. Once you have that down, I would recommend trying it on a boggart. Then once you have mastered that, you are almost ready to fight a real dementor.