I have a 1 speed road bike and I want to put gears on it how would I do so?

I have a 1 speed bike, its a genesis road bike. But i want to add gears on it so its easy for me to get up hills and go faster down them. Im lookin maybe an 18 speed. How would I do so? Also My bike is a flip flop, idk what that means but idk if it will help at all