You muscles are not going to do much on their own unless you do something about it. Basically in order to gain any new muscle you literally have to force them to grow. It is important that you don't fall for the quick fix muscle gaining pills or drinks, as tempting as it might be it's not going to do you any good in the long run. Besides, synthetic substances can have long term ill effects on you and your body from dehydration to dependency. So it is better to do it the traditional way, lots of hard work and a good diet.
In order to force your muscles to grow you will need a very high intensity resistance workout program. You need to learn to work out certain groups of muscles on one day and do another group of muscles on the next day. For instance do the shoulder area on a Monday, do your legs and hamstrings on a Wednesday and do your arms and chest on a Friday. This gives all of the different areas attention they need and rest they also need between workouts.
It is not the weight that is important, especially in the beginning. It's how many reps and how slow you perform those reps that are important in forcing your muscles to grow in a healthy manner. You continue to use a certain weight at the same reps and speed until it seems to easy to do any longer. This is when you increase the weight but continue the same amount of reps and the same slow speed in which you began with.
Make sure that you provide your body with plenty of lean protein. Your body needs adequate nutrients in order for it to maintain the muscle mass that you are adding to your body. Protein is vital for muscle growth. So eating whey protein, eggs and meat like fish, chicken and very lean beef are great sources. You can also get your protein source from milk, cheese and of course yogurt. It's a good thing to start the body off with some protein in the mornings and then slow down the digestive system by having a dairy product before going to bed at night.
You will need to make sure that you have some carbohydrates in your diet as well. Carbohydrates will serve as an energy source for your muscle growth. If there are not enough carbs in the body the body won't have enough energy to produce more muscle. In the morning you should probably include some simple carbs for some quick energy like some fruit juice but most of the time your diet should include complex carbohydrates like rice, potatoes, oats, whole grains and pasta. You will also find that corn, broccoli and beans are also complex carbohydrates and these should also be included in your diet.
You also need to make sure that you have enough omega-3 and omega-6 fatty acids in your body. You can get both of these by simply including avocados, fish, soy, walnuts, fish oil, flax oil and sunflower oil in your diet. Make sure however that you keep your intake of saturated fats down to 10 percent of your calorie intake in a day.