If you are conscious for looking then you should be conscious for your diet
as well. Diet dosen't mean you have to eat thereputic food. Weight gain is a gradual process. You have to keep patience and 3-4 kgs weight gain is a safe weight gain which includes muscle mass not fat. So first of all you start taking small frequent meals. Eat at right time. Increase your fluid intake. These are the general tips but everyone has different body type so same advice can't be given to others and for customised diet chart
a clinical dietician needs to take medical history, diet recall, your activity level schedule as well and then can plan specified diet chart for you. Which is based on your likes and dislikes also.