In this Instructable, I take you through one of the processes of gaining muscle mass (there are many). This method involves resistance (weight) training and nutrition. This style is termed "Natural Bodybuilding" because of the fact that it doesn't involve supplements of any kind (i.e. anything recommended in the nutrition section can be found at any normal grocery store).
First and foremost I'd like to stress the importance of safety for people ready to begin a training program. As with any form of exercise, it is greatly advised to consult with a doctor in order to ensure your safety while putting your body under new (and tougher) conditions.
With that said, congratulations on having this much drive to begin a training regimen, and let's begin.
Step 1: Knowledge is Power
This is by far the most important information in this Instructable:
You know your body better than anyone else
Every body is different
I would recommend strongly against taking any one source of information as truth (including this Instructable!). What works for some other people may not work for you. Self-knowledge is VERY valuable when you are working out. Much of the time that building muscle and strength is used figuring what works for you. As you advance in your training, you will learn more and more about your body, its strengths and weaknesses and how it will respond to different exercises, weights, rest periods, and other variables.
I would recommend having (or acquiring) some knowledge on nutrition, anatomy, and resistance training in general. These can be found in workout manuals (big books which look like telephone books). These are good for reference and getting new ideas for different workouts and exercises to put in your routine.