Gaining weight is all about eating more calories than you burn and doing it over a long period of time. When you first start changing your diet, it'll be hard to find the foods that will add enough calories to allow you to gain weight. Breakfast, lunch, dinner and snacks in between meals have to filled with calories in order to gain weight.
The cliche that breakfast is the most important meal of the day is true. If you go to sleep around 10pm, your last meal was probably around 7 or 8. When you wake up at 6am, you've been fasting for almost 12 hours or half a day. This isn't good for weight gain.
When you're trying to gain weight, you need to eat a lot and often. Fasting for 12 hours isn't ideal. Since avoiding this is hard and impractical, you can take steps to minimize the impact.
This is where a nutritious and calorie filled breakfast comes in. Try to eat a meal that is high in protein and carbohydrates while low in fat. Don't totally avoid fat since it plays an important role in our diets. Some good choices for breakfast include eggs, cereal, oatmeal, orange juice and milk (regular or chocolate). Have more than one choice for breakfast since your body is begging you for energy.
You have plenty of options for lunch. For many people, lunch is a break from work and means going outside to the closest restaurant. If you're trying to quickly gain weight then there's nothing wrong with this approach, but there are better options. If you aren't in the mood to cook, consider lunch as a big snack. Eat some sandwiches, nuts, fruits and have some (non-alcoholic) drinks. The calories will add up quickly.
Dinner, like breakfast is also an important meal. While you sleep your body repairs the damage you caused from working out and doing daily chores. To do this, your body needs energy (carbs and fat) and amino acids, the building blocks of muscles (protein).
If you eat a dinner that's high in calories, your body will have the resources it needs to rebuild, repair and recover. If not, you won't gain muscle as quickly as you'd like. Meals for dinner are limited to only what you can cook up and imagine. Pizza, chili, pasta, fish, chicken, steak, potatoes and lasagna are all good options as well as the hundreds that aren't mentioned here.
Snacks are very important even though they aren't full meals . If you have 3 snacks per day at 200 calories (a very low estimation) each, it adds up to 600 calories which can be the difference between a 2400 and 3000 calorie day. Snacks are an excellent way to add calories to your daily diet. Nuts, sandwiches, drinks and fruits are all good ideas.
The Bottom Line
No matter what kind of food you like, the most important point to remember is that gaining weight can only be accomplished by taking in a lot of calories and eating often.