Most Helpful Guy
What kind of weight do you want to gain? If you want curves, lifting is going to be the best way to get them, and so any food with a lot of protein is the way to go. Tuna, black beans, chicken breast, edamame, low-fat string cheese, whey protein powder (pre and post workout), casein protein power (before bed), salmon, eggs, peanut butter, lean steak, almonds. Then hit the barbells and get some squats in. If you can't handle that much weight, just start with the bar. Watch videos on how to do it, or just get a cute guy to show you proper form while at the gym. If you hate the gym, you can get a kettlebell or a dumbbell set and do goblet squats.
If you just want to bulk up and gain a few extra pounds, then carbs are the fastest way. Eat a lot of pasta and rice. Avoid sugary foods and junk food as they don't have much nutrition outside of the carbs.
Here are a few guides:
If you just want to pack on fat or you just want an overall comprehensive weight gain: