Anatomically north of where the lips meet is the clitoral hood, kind of the female equivalent of the male foreskin, which covers the clitoris. The degree to which the clitoris is covered will vary from woman to woman, but with arousal, the clit will swell with blood and poke out.
Take your finger out of her pussy (ideally covered with her own natural lubrication) while fingering her, and run it up to just past where the lips meet. Gently rub the little bump you find there. She'll be calling you the Clit Commander.