How do I know where her clitoris is when I am going to finger my girlfriend?

This little bud is to be found at the top of the inner lips, where they come together. 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, these will touch her outer lips. When you push these a little bit aside with your fingers, you have a good chance that they will touch the clitoris. Use two or three fingers instead of one: this increases the chance of hitting it. But of course you can also ask your girlfriend. She can show you where to touch her. An easy way to do this is let her touch herself, covering her hand with yours. If you get the just of it, you can turn it around and ask her to guide your hand.