Whether you are interested in body building or weight gain, the goal is basically the same, putting more muscle on to your frame.

To gain more muscle your will need to:
Sounds pretty simple, well not so fast.
Lets tackle the training first, there are many more factors to consider such as:
Muscle balance always amuses me, in the gym you will notice those people that focus solely on their chest and biceps. What about the legs and the other 8 muscle groups, not only do these people look ammusing, they are at serious risk of injury as their bodies are so out of balance.
Ego training is also worth a mention here, the guys lifting as much weight as possible, using terrible technique and putting themselves at risk of injury.
If you get the repetition range wrong, you will look more like a powerlifter than a body builder.
You will also need advise on how to get your nutrition right. Most people who are underweight do not eat enough food in the first insatnce.
Give Roland a call for an imformal chat about your Body Building / Weight Gain aspirations, with 24 years of weight lifting experience, you will be in good hands.