To increase your muscle size, or mass, you need to focus on your training and your nutritional intake.
To train correctly you need to structure your training plan. Firstly make one workout for each body area you want to add size to. Then find 2-3 exercises for each workout. Try to focus on exercises that hit the same body area in different ways, for example barbell bench press, chest flyes and incline dumbbell bench press.
During the sessions the number of reps, sets and other training factors all depend on your fitness level. Do between 3-6 sets, 12-6 reps, 30-90 seconds rest and 67-85% of your 1 rep max for each exercise.
To give your body enough fuel to grow your muscles you will need to increase your calorie intake. On average people need to increase the number of calories they eat per day by around 20%. However you will need to experiment yourself, as this is only a rough guide. Try to find how many extra calories you can eat before you begin to put on extra fat. Just before this is the optimal calorie increase for building muscle but not adding fat.
Finally focus on quality food. Just because you are trying to consume extra calories does not mean that you should binge out on chocolate and cheeseburgers. Good food will allow your muscles to grow quicker. Try to eat lots of protein, good fats (unsaturated) and complex carbohydrates.