My girlfriend is crying how do I calm her down?

Just hug her everytime she is crying.. Then when she calm you can ask her for the reason why she cried? Be patient!!!

Maybe she had a nightmare, where she saw you cheating on her .. Sometimes the loudest words are the silent ones... Give her a kiss.. That could also calm her down..:P

Validate what she is saying by being there for her. Sometimes you don;t have to say anything depending on if you caused her to cry or not! Then you should tell her  you're sorry and mean it. Then allow her to talk it out with you without interuption. By doing that she will help resolve her own feelings about things.

Tell her you'll take care of it :D

Listen to her and care what she is saying. Tell her that you care and that you love her.

Hold her hand and tell her you're there for her. Hug her and let her know she's not alone in whatever is bothering her. It's pretty easy!

Ask her what the problem is causing her pain
Beyond that..... Grab some blanket and try to get some sleep

Just shock her with your instincts