What can I do to treat anorexia nervosa?

Support & Knowledge . Once someone w/anorexia starts his/her recovery -starts reversing effects of malnutrition by eating a balanced diet 2get the nutrients the body needs & 2get hormones back in balance- s/he can start 2heal. But recovery can be very tough though & relapse is very common. Really helps 2have good medical/mental health team, 2know a lot re: anorexia; & 2have strong support net. http://www.anad.org.
Accept weight gain. Weight restoration, while not everything, is the first, critical step towards recovery. No psychological processing/healing is possible if a patient remains underweight and the brain malnourished.
Seek Help. As anorxia nervosa can be lethal, which means you could actually die, my best advice is to not treat yourself, but rather to seek treatment from a person who specializes in this area. If i knew how to tell you treat yourself, i would. I can only assume you must have tried some stuff on your own already, which is why you are now posting here. Treatment helps and those who are not treated risk dying.