Do you have difficulty gaining weight? If you are a "hardgainer," try looking at programs designed for your type.
They're generally about focusing on big important exercises, avoiding overtraining, and eating a ton of healthy food.
Generally, keep it simple:
You're going to want to bump up your caloric intake by at least 500 calories/day above your current weight's average. Take a protein supplement as a part of that to get 25 or 30 extra grams.
Stick to the basic movements, and do full-body workouts:
- Pushup/chest press/chest dip
- Pullup/row
- Shoulder press
- Squat/lunge/deadlift
People will disagree with me, but avoid tricep and bicep exercises. If you focus more on the chest/back/shoulder movements above, your arms will get enough of a workout. If you feel like you should be doing 1 to 3 sets of curls and pulldowns...how about doing 1 to 3 more sets of pushups, pullups, and presses, champ? I'd only do tricep/bicep work for definition or if I were doing bodybuilding training for competition. Most of your muscle gain will be in your legs, chest, and back anyway. Focus on those, and the rest will follow. The exercises above use your arms well enough.
Get lots of rest. Don't workout more than 3/week, and never back-to-back days.
Drink lots of water. Take a multivitamin unless you're a health nut.