Needing to gain weight is not a green signal to consume junk food that is full of useless fats. Of course, you will need to consume more calories than you burn, but be sure that you eat only healthy calories. Eat food rich in omega-3 essential fatty acids. This will be found in fish like tuna, salmon as well as some nuts. Fruit, vegetables, whole grains, legumes, lean meats and poultry can also be part of your diet plan. If you generally lack appetite, then spread out your calorie intake in five or six small meals. Protein supplements and shakes taken in between meals will also help you gain weight.