How can i become fat? i'm thin and low weight.

Underweight: First you need to see your doctor to do some tests to determine if there is a hormonal cause such as hyperthyroidism or another health condition causing that. If all is well do regular exercise to build muscle mass. Try high protein diet try supplements such as ensure or boost.

Eat more calories.: I assume that when you say "fat" you mean to gain weight. Gaining weight can be done in a healthy way. To gain more muscle try exercising and lifting weights while focusing on a protein rich diet. Eating too much fat or too many carbohydrates can be bad for your health.