What to eat while you are working out and want to gain weight?

I started aerobic cles to strengthen my body. However, i don't wanna lose any weight because I am already underweight. I wanna gain about 4 to 5 kgs. what kind of food should I eat in order to help me reach my goal? I don't want to focus on high fat unhealthy kinds of food. I wanna be cellulite free when I gain it.