I am not sure whether you actually want to be "fat" or look muscular and fit. You are actually looking towards having a more balanced and attractive physique.
Getting muscular or getting thin, both requires effort and time and does not happen in a couple of months. You need to have patience and track your progress on a weekly basis. Diet and workout both play a role here. Give yourself at least 4 months to see a visible change.
Do not adopt unhealthy eating habits or laziness to get fat. This will do you a lot of harm and invite diseases and weakness.
Eat more, but eat healthy foods, full of nutrition. Like adding more protein, more fibre and fruits/veggies. Add carbs too but be balanced with sugars and fats. Add jaggery/brown sugar and olive oil to your diet. Add almonds, raisins and walnuts. Just because you want to increase your body mass, do not load yourself with sugary and fatty stuff.
Include weight training, free body exercises to your routine. The ratio of weight training to cardio should be more in terms of time devoted, like 40 minutes of weights followed by 15 mins of cardio. Do weight training after warming up and cardio as a last exercise before cool down. Do squats and pushups as they increase total body strength. If you include whey protein in your diet it will be better, but its not mandatory if you are getting enough protein elsewhere. You should have a before and after workout diet. Before workout, have fibrous carbs and after workout have protein and fast absorbing carbs.
Hope it helps. This is from personal experience and watching other people do the same.