how to learn American sign language?

My answer may not be as helpful because I do not live in the States, but there might be courses available at your local adult education centre or local deaf school/centre. See if you can also find some sort of Deaf community web page and ask someone who knows where courses are on offer close to you.

I am learning German SL and enrolled for a course through something called Volkshochschule (basically adult education schools)

SL is a really nice thing to learn because it helps you communicate more visually day to day and one also learns to read the entire body.

Of course, not all deaf people sign. My uncle was as deaf as a post and attended a normal school. he learnt to read people's lips and functioned very well in society.

If you like, train yourself to mouth words clearly e.g. read aloud with a mirror in front of your and shape each word clearly. Also, check out youtube - there might be something useful there.

The best way to learn ASL is having a deaf person teach you the proper way to sign... If not then try to hang out in places the deaf community gets together. I agree completely with what Ally said and not only is it necessary to know ASL words but you also need to learn how to properly use gestures and classifiers when communicating with the deaf community (the more you hang with them or are in contact with them the more you learn on how to appropriately use certain signs and gestures.) The link below is very helpful. Just keep in mind that there may be some slight changes in some signs, depending in what region of the US some signs are slightly different.... Just like any language- for example, i speak fluent Spanish and at times when speaking with individuals from Spain, Puerto Rico or South America i need some clarification because they may use an expression or word I'm not too familiar with... same with ASL

Definitely get books on American Sign Language but watch out because a lot of sign language books are really about Pigin Sign English or Signed Exact English, not true ASL. Also, be sure to invest in a good ASL dictionary. I have one of over 5,000 signs that I got from the online site Amazon. And, if you can't find Deaf or hard of hearing people in your area, try going online to places like Youtube. Two web sites that I recommend for free ASL instruction are ASL University (you can Google it), and aslpro.com. Good luck!

Some about the sign language

http://en.wikipedia.org/wiki/Sign_language

Official American Sign language Reference

http://www.lifeprint.com/

You can get yourself a sign language dictionary and self-learn.. they have pictures in them and it isn't all that difficult to pick up. To improve at a faster rate you should probably find someone who's hearing-impaired to practise with though. Have fun! =)