Since we mentioned increasing size and weight from the get-go, we're going to address it first. But remember, we're not trying to compel you readers to grow championship-caliber muscles; leave that to the steroid professionals. We're simply trying to advocate an ideal body figure worthy of everyone's glances.
Having said that, let's try to picture something out. Your body acts like a bank, with the corresponding deposit/withdrawal systems in it. You deposit "monies" and savings into it, making your account balance grow bigger. You make it smaller by withdrawing from the account. I'm afraid the illustration isn't too accurate, but I’d like to assume that you get the picture.
The basic problem of the thin person is that he/she does not put enough into the account. At the end of the day, the person withdraws more than what is deposited. That's what usually happens to thin people: spending more than what they've taken in. When that happens, the body will turn to its fat reserves more often, emaciating the person and oftentimes depleting the body's natural store of trace nutrients. That's when he/she becomes undernourished.
The solution is obvious, eat more and eat right. Milk is one of the most beneficial foods for adding weight. So does a lot of protein and dairy products. A lot of vegetables, fruits, fiber, and fluids will help too. Aside from those, put a lot of love to your body by subjecting it to exercise.
When starting out gym sessions to bulk up, a person is usually recommended to find the maximum ideal weight he/she can carry with a minimum number of repetitions. For example, an underweight, skinny guy might do a bench press six times with a 40-pound barbell. Or he might be compelled to do a standing dumb bell row with 15-pound dumb bells in each hand for only 8 repetitions. The varieties in which to start are too many to count.
Now you might wonder why so much weight is introduced while working the muscle groups with so few repetitions. The basic idea here is to stimulate the muscles. The thin man starts out with less muscle fibers, less muscular density, and -- I'd like to add -- puny muscle sizes. In order to "grow" muscle, we need to "damage" muscle fibers by stimulating it with really heavy weights. When this is achieved, the muscles affected will repair torn muscle fibers, building more in the process, and consequently increasing in mass!
Hence, that gnawing pain that you feel after a strenuous workout. If you experience this, then smile! Know that you have invested something worthwhile with your time. If not, then obviously you haven't made too much effort.