link you can put in your height in feet and inches than it gives your BMI number and on the chart it shows you where you are in the underweight-obesity. from there you can adjust the weight number to make that your goal.
I just did 5'9 at 126lbs and that's barely over underweight.
To be at your maximum health, their is an equation.
For a female, 100 lbs at 5'0 and +4-5 lbs for every inch taller. (try to keep it muscle.) You should be around 136-145lbs minimum. This is best so as to not put to much stress on organs by being too thin. Really, it depends on your body shape. If you got curves, work them girl! You'd be ideal for a model by the way. As long as you feel healthy, nothing else should matter. If you honestly feel like you are overweight and feel tired a lot, gain muscle. If you feel too thin, work out! Eat food:) It's yummy! Let your body do the talking. The sympathetic part of your brain will kick in if you're too thin or overweight and is causing damage(most people ignore it). Oh and try yoga. It finds your balance. Goodluck <3