While exercising is important for building muscle, you'll also want a good diet that's packed with a ton of good foods.  Follow this guide and I'll teach you about the best foods to eat to gain muscle as quickly as possible.

The first thing you need to know if you're skinny and trying to gain muscle is that you're not eating enough.  I can't be more blunt than that.  If you're not gaining any sort of weight, then you're simply burning more calories than you're consuming or you're at least at baseline.  In order to gain weight, you'll need to eat about 300 more calories than you burn, although more is never an issue for skinny guys.  While it's important to eat a lot of calories, you'll want to make sure they're good calories and not completely useless.  

When you decide on a diet, you'll want to take one that has a ton of foods that are high in protein.  Protein is used to help repair your muscles after you've worked out and will allow you to gain weight very quickly.  It's important to note, however, that protein isn't any good for energy and shouldn't really be taken before a workout because it'll just digest slowly and be useless while you're working out.  The best foods to eat before working out are those that are high in cards, like pasta.

That brings me to another point: carbohydrates are by no means a bad thing.  You'll actually need to eat a lot of them if you want to gain weight - they are the main source of energy for your body and will help you run at maximum efficiency.  Carbs should actually make up the majority of your muscle building diet - about 50% of the calories you consume should actually be from carbs.  Some great high carb foods are pasta and cereal, both of which will give you plenty of energy.

Protein, like I said, is very important.  The best foods to eat for protein are steak, chicken, and tuna fish.  Tuna is probably the most cost-effective way to get a good amount of protein, but I would avoid eating too much of it due to the mercury content.  While it won't have much of an effect in small amounts, eating a ton of tuna could cause some degree of heavy metal poisoning - something you don't want to go through.

Finally, you'll want to get some fats too.  Fats are important because they promote neural health and activity.  About 15% of your calories should come from fats, but make sure they're healthy ones.  Good sources of fat include olive oil, peanut butter, and fish.  All of these will give you healthy fat that'll ensure you gain muscle quickly.