Significantly Increase Your Protein
If you are looking for a type of food to add more calories in your diet, protein is always a good option. The general rule is a gram per pound of body weight; however if you are looking to gain muscle you are going to be gaining some weight. Try adding in extra protein based on your desired weight. You want to fuel your body for what you want to achieve.
Up Your Weight
If you are specifically focusing on muscle gain, focus on heavier weights as opposed to more reps. There is a place for each in your training but when focusing on muscle gain heavier weights are important. As you begin to gain strength you can begin upping your reps, but just make sure to be careful and properly warm up before any weight lifting, especially when increasing the amount of weight.