Before doing anything else, I think it's important to figure out if she's actually pregnant. Confirm with a pregnancy test.
Then, if she isn't you can help her get better information about birth control and making better, more responsible choices. In many responses, Rob has cited this information: "The web page below has nine links at the bottom. If you understand all of the information on those pages, you will clearly understand how it works and the risks of different behaviors."
Planned Parenthood:
http://www.plannedparenthood.org/info-for-teens/pregnancy/am-pregnant-33831.asp
If, she is... that's a whole other ball game, but no point in getting worked up about that yet. Get the test first.