The clitoris is the most sensitive part of the vulva. If you rest the palm of your hand on her mons (the part that is covered with hair) and you let your fingers point down, the clitoris is beneath your middle finger.
If you finger a girl don’t use one finger, unless you know exactly what you are doing. Using two or three increases the chance of hitting the hot spot. You can move your fingers up and down or make a circular motion.
Some girls like it if the clitoris is stimulated directly, but others find this too much. They prefer indirect stimulation. This can be done by touching the skin next the clitoris or the inner lips or by manipulating the clitoral hood.
As the girl becomes more and more excited, you might want to increase the pressure of your strokes and increase the speed.
Some want the same kind of movement as long as possible, while others prefer some variety. So ask her what she wants.