What can i do to gain weight as fast as possible?

A good diet.: Try eating a good, healthy, well balanced diet, focusing on proteins and carbohydrates. Try to minimize your fat intake. Begin to exercise, focusing on muscle building. As you build muscle, you'll gain weight from muscle growth, rather than fat.

Listen to your body: You probably want to gain muscle mass. That take healthy habits. Avoid tobacco, alcohol, junk food. Get a good night sleep, eat 6 healthy meals a day with high quality protein, fresh vegetables and fruits, whole grains and good fats. Exercise through the day but don't over-train. Manage stress skillfully. Relax and enjoy life.