If its muscle you are trying to add, the ground rules for us skinny guys is to eat 1 gram of protein for every kilo of our goal weight. Also make sure you are getting the calories required at that goal weight. Other than that, focus on all around nutrition so your body can efficiently use that protein. Also you need to start lifting heavy. Short reps and slow lifts. The Exomorph body type (naturally thin) needs brief, intense exercise to put on muscle. I'm 5'9" and if I do nothing but normal daily activities, I shrink down to 140lb or less. Working out 3 days a week lifting as heavy as I can, lifting 3 sets of 5 reps to failure (if you can do 7 reps, increase the weight. If you can only do three, reduce the weight) and focusing on a few complex motions like squats and clean&press rather than bicep curls, etc. This got me over 160lb and chiseled like a Greek statue. I wasn't going for excessive bulk, just functional strength and aesthetics. I've lost a lot of it because of life changes, but I know what I can do. If you just want to add some mass to fill you out and don't mind most of it being fat, any combination of carbs and fat together is ideal (not exactly healthy) though extreme exomorphs like me will just sweat it off in our sleep. Good luck.
