There are several different types of sign language, you should really find out which one they use to communicate.
Those links that motoamd provided were good, but those are for ASL only. There's also pigeon sign which is where they fingerspell every word really really fast, and signed english which is similar to ASL with slightly different signs (most of the signs having the handshape of the first letter of the word you're signing), and the grammar is identical to english.
Also, sign language is a rapidly evolving language, so unless her parents are active in the deaf community, they may not recognize some of the "new" signs that you'll learn.
I wouldn't really reccomend just trying to teach yourself, you'll learn a lot more if you take a class on it, and there's a good chance your teacher will be familliar with all the ins and outs, if there's several common ways that people sign a word, they'll show you all of them (whereas those sites just show the most common one).
I'm currently taking American Sign Language II at Lansing Community College. If you have any more questions feel free to ask
