More calories is the simple answer. When you consume more calories than you burn in a given day, you will gain weight slowly but surely. I recommend downloading a calorie counter app like "Lose It!" or "MyFitnessPal" to keep a direct track of what and how much you are eating. It can be a hassle but that alone will tell us a lot.
I have a friend who has been a bachelor his whole life and now have turned 32 this month. He was unable to face any girl due to his consciousness about his thin personality. A colleague of his suggested him to visit their website and join online dating, and also advised him to gain some weight till he meet the girl in person. Suggestions for gaining weight will be very helpful. Thank you in advance.