Kayle is a versatile champion who can play in every lane. (Top, Bot, Jung and Mid) She damages champions mostly with her basic attacks, enhanced by her E, "Righteous Fury" and her Q can be used to deal damage and slow enemies. Other than offensive abilities, her W and Ult are pretty good defensive abilities. Her W heals an ally or herself and speeding them up.
Against a melee top laner like Garen, you won't have a bad time because her E will make you a ranged champion for 10 seconds, deal AoE damage and deal bonus AP damage.

Against Melee Champions

Against melee champions, you should stay away from them and engage with your E. If they catch you, slow them with your Q and use W on yourself. If you go low on health, use your Ult on yourself. This can be gamechanging.

Against Ranged Champions

Against ranged champions, use your W more often. If you are against champions with skillshots, try to dodge them and engage when their skills are gone. If you are against non-skillshot champions with high AS, try to outplay them with your E and Q. If you go low on health, don't forget to Ult.