The secret to building muscle is directly proportional to getting stronger. Muscle builds up when you lift up something heavy regularly. The heavier the object you lift the more muscles you get. If you lift the same weight over and over again you do not gain any muscles you just maintain the muscles you have now. You will know that you are gaining or building up muscles when the weight that you lift is progressive. But take note do not tear your muscles. Do not shock your muscles it will only get you sore and injured. Start small keep a progressive pace do not immediately lift something that will make you hit failure just find a weight you can handle and gradually increase it as time goes by. Do not confuse your muscles with changing workouts every time you work out you will gain nothing from that but a sore body and wasted time.
Find a workout that is suited for you if you are really skinny and do not want to build muscle because you are afraid that it will make you look even less proportional that you already do find a work out that makes every part of your body work and uses effort. If you want over all muscle then you should do an over all workout not just an upper body workout. Everyone can gain muscle if they just want it enough even hard gainers. Hard gainers are those who have a lanky body and are just thin by genetics. Building muscle is very simple but it looks hard because of the various workout machines that we see. The most important key to building muscle and maintaining it is discipline. You have to discipline yourself in order for you to maintain the body you want to have and to be consistent in your schedule in regards to going to the gym.
Do not over work yourself by going to the gym almost everyday, three times a week is enough for you develop and maintain muscles. You also have to eat, eat a lot of unprocessed food this means bye-bye fast food and junk food and start eating what the old generation would classify as food. You have to maintain an intake of at least two hundred grams of protein a day for your body to have energy for working out. Lastly hydrate since you will be sweating a lot you also have to drink a lot of water.