Clean up your diet and you will see changes straight away. Try to avoid the overweight/underweight hysteria. I think such labels are baseless most of the time, and can easily become the psychological fuel for eating disorders.
What does your diet look like? What is your physical activity level?
I am 5'11". Six years ago, I weighed in at 125lb. I was a runner.
Today, I weigh in at 150lb. I can probably help by providing some ideas. I like Ngibby's response as well.. Answer my questions, please.