17 year old skinny male

I am 17 years old, 5 feet ten inches, I weigh 125 pounds, and I have a very fast metabolism. I know I am a little under weight but I want to build muscle. Mostly I want to build for aesthetic reasons (abs, chest, arms). I have been doing the 8 minute ab work out(the one on YouTube), and I am starting to do chest and arm exercises, and have been trying to eat as much good food as possible. Are there any workouts that one would suggest for me? I don\\\'t need to lose weight, just gain muscle. Thanks Also, if my muscles shake while I am doing a sit up, does this mean they are just not strong enough to stabilize my weight yet?

im doing them too and the results are impressive