You should probably start with your pediatrician, who may have some resources he or she knows of. Many community colleges off courses in ASL. And you can use your internet search engine to see if there are any classes in your area. I am willing to bet there are also some starter videos on YouTube.
While you're in our community, you can get some great feedback not only on where to find resources, but also other questions you may have about your child's health, growth and development on our Parenting Community . Our pediatrician there, Dr. Roy Benaroch, is super.
I have been reading a lot lately about Executive Function in regards to both my son and my college-age daughter. From what I have read, some kids can fake it for quite a long time, and then suddenly realize they are in over their heads and can't organize themselves.
Check out that link and then I'd recommend posting to Dr. Roy Benaroch on our Parenting community . Though your daughter is older, she's still your kid.
Have you tried getting her in to see a psychologist for evaluation?