I'm an18 year old female with a height of 5ft 4, what weight should I be?

BMI at 5"4" To achieve a BMI < 25 which is normal weight you should weigh between 120 -140 lbs closer to 120 -125.
100 + 20 = 120. There is a big range for healthy weights for a given height. A ballpark number is 100 pounds plus 5 pounds for each inch over 5 feet. Many women who are 64 inches tall will weigh between 110 and 130 pounds.