Get married. It'll put 20 pounds on no problem 
seriously, talk with a real doctor. See if you are seriously underweight or not. I knew someone who looked like a skeleton who seriously needed to eat, but if that is you in the avatar you look fine.
As for putting on weight. Moderate weight bearing exercises would help build muscle rather than just fat. Eat about 500 calories per day more than you need to maintain your weight. So you could go to a website like my-calorie-counter.com and use that as a food journal, and monitor your weight. If you are staying steady than add 500 calories per day. That will help you gain 1lb/week. If you are exercising building muscle you'll turn out great rather than turning into flab because you downed a whole bunch of protein shakes and sat on your butt.








