Hi, read the posts i did in theese two: http://www.medhelp.org/posts/Eating-Disorders/Weight-loss/show/1377871
, that should provide some information, If you have any questions please feel free to ask, I've had the same problem as you and I am an ex bodybuilder/triathlete
Talk to a Certified Personal Trainer. They can help with adding lots of protein to your diet, healthy calories, and weight gaining protein shakes and lifting weights. Stay away from cardio. Make sure you are ok physically , that something isn't keeping you from gaining otherwise the above shoudl do the trick.