If you want any lasting gains, I suggest you ditch the word "fast".
For most of us, getting a body you are proud of takes time, and is a long and arduous process that requires constant motivation, training and determination. Though I am the opposite end of the spectrum, and can put on fat and muscle very easily, the ideas are the same:
1. Have a healthy, nutritious diet with lots of lean protein, fibrous veggies, fruits, nuts, healthy fats. This is not just for working out, but for a longer, healthier life. The only difference for you is that you need to eat more of it.
2. Work out consistently and push yourself constantly. As a hard gainer, I can imagine the muscle building process will require even greater intensity and consistency, not just with diet, but with your weight training.
The bottom line is that you must work hard. There is nothing "fast" about putting on lbs of muscle and getting ripped. You are lucky that fat hates sticking to your body, so you will have very strong definition immediately, and can be shredded and maintain it easier. Eat a lot, work out hard and consistantly, and stay motivated, and its only a matter of time.
