Move your finger in and out of the vagina. Also you can insert you finger (palm up) about an inch (give or take) into the vagina until you feel a spongy surface. Once you have found that make a "come her" motion with your inserted finger. That, as you will soon find, is the Gräfenberg or G spot. And don't neglect the clit.
For a more advanced night of fun, you might also might try moving your thumb across your clit as you work the g spot.