First off, accept the fact that your genetics developed you muscular structure different than other people. Plus your muscle definition is going to look different than others also. (IE: Some people have shorter muscles while other have longer muscles...take a good look at calf muscles length.)
Second: You have to accept the fact that your body type will require a different exercise routine than what another person may have. (IE: "Ectomorph," "Mesomorph," and "Endomorph")
Personally speaking, my body type is "Mesomorph" with long muscles. The training routine that works for my body may not work for someone with an "Ectomorph" or "Endomorph" body type.
Finally, knowing what you body type will point you in the right direction as you continue your research. Search for bodybuilding articles regarding the body types mentioned above.(Ectomorph, Mesomorph, Endomorph) Going off of what you have stated about yourself...I am guessing your body type is "Ectomorph."
Remember there are not any finite work out guidelines that will work for every body type. Slow progress is still progress.
Please don't. That's over-rated, I'm sure you look fine just the way you are. (:
Eat Lot and work out
First you can't build muscle without fat so carboload my friend. Make sure you work your whole upper body. Shoulders back pecs abs TRICEPS and biceps. I capitalized triceps because most people focus on biceps and trics are the bigger muscles in your arm. You can always cheat and steroid the crap but that's pathetic. Just got to keep working and perfection is all about perception. You'll never be perfect
First realize that more of what GeorgeMack said is a lie. You are not a "hard gainer". You simply don't eat as much as others, or you're more wirey/active than others. Either way, the solution isn't acceptance. Don't waste your time researching into genetics, when it's only a factor in the top 0.1% of competitive athletes.
The solution is-- more calories, more resistance training (which is proven to have the same effect on everyone-- no special exercises needed).
You should lift 3-4 sets of 8-12 reps x 3 exercises per muscle group. That translates to about 100 reps 3x a week per muscle group. This is the body building method-- high volume resistance training (hypertrophy training). Drink half a gallon of milk a day plus 2 protein drinks (morning&night or post workout & an hour before bed). Do this consistently for 90 days with no workouts missed, consistent protein and I guarantee you results you'll love. I've gone from a 125lb cross country runner in high school to 165lb running as fast as I was at 125lb. The key is consistency. Consistent exercise, consistent protein = consistent results.
basically what the girl just said, she said it all but its hard work. make sure it a healthy bulk and not a bad bulk, like eat a lot of good fats food rather then bad ones, you want as much protein as possible too