To Gain Weight

If your body type is such that you can not easily gain weight or add muscle (ectomorph), a conventional diet and routine won't help you much.  There's a different set of rules for this type of body, which is usually the tall lean type.

1. Eat as much as possible and don't cut down on fat.  With your body type (especially if you're young) you burn a tremendous amount of calories, and fat is an excellent source of energy. Eat lots of protein (meats, fish, dairy, whey protein, grains), and a moderate amount of complex carbs (vegetables, rice, pasta, grains). Eat often, including before bed. Some even wake up in the middle of the night to eat, but that is not necessary.

2. Minimize aerobics!  The last thing your hard earned muscle needs is to be burned up while sitting on a stationary bike.

3. Train using mostly low sets and reps with heavy weight. (Every 3-4 weeks, switch to higher reps (10-15) for a week or two).    Do 3 - 6 sets per body part on a double or triple split system, training 4 - 5 times per week. Train each body part every 4 - 6 days,  use a weight that allows 6-10 reps per set, with 2 - 5 minutes between sets. (Depending on the bodypart --> less time for the smaller body parts, more time for heavy leg work or deadlifts). The workouts should be completed in under an hour. A half hour per body part is a good rule of thumb (20 minutes if doing 3 body parts).

4. If you still don't make good gains, take a few days off,  go on this routine for about 6 weeks, then go back to to the lower set/rep training.   Alternate these every 6-8 weeks.

Follow these and strength and growth will come to you. You may never get really big but you will definitely make gains.