Some people are blessed (though sometimes they don;t think so) with a super fast metabolism.  that means youburn calories quickly, making the process of gaining weight or putting weight on really difficult.

Try to add healthy fats like nuts, avocado, olive oil, canola oil to your diet.

Increase your whole grain carbohydrate intake with healthy cereals (have a double portion), high protein pasta (Barilla brand), rice, breads.

Have a smoothie daily that has flaxseed (healthy fat), protein powder, yogurt, fruits (there are lots of recipes online)

Make sure you eat adequate protein - fish, eggs and egg whites, beans, white meat chicken and turkey, peanut butter.