Starting off as a skinny guy myself, gaining weight has always been a challenge for me. However, through research I learned that gaining weight and building muscle wasn't as complicated as people were making it out to be. You don't need to pay anyone to teach you this information -- I'm going to give you everything you need right here.
How do we gain weight? Well it's pretty simple. You eat more calories than you burn. A lot of skinny guys say they eat a lot (myself included) but at the end of the day, they aren't breaking the laws of thermodynamics. In other words, you need to eat more food.
Obviously the real challenge is fitting in a high number of calories when eating at a maintenance level is already a chore. The best trick you can use is to eat foods that are loaded with calories, or calorically dense foods. There are lots of foods that are calorically dense (but are maybe not so good for you), like ice cream and pizza. However most people like their foods to be nutrient dense as well. Here are my top 5 foods that make gaining weight easier:
Peanut Butter
Rice
Nuts
Whole Milk
Avocado
These are all foods that have lots of calories, versatile, and are very palatable. Incorporating more of these into your diet will make it far easier to reach a caloric surplus and then gain weight.
Good luck to all the hard gainers out there!
Peanut butter and whole milk worked really well for me. Very easy to make a lot of milkshakes with those