What is the good range of weight for an 18 year old female at about 5'8?

About 145 pounds: Normal women are a wide variety of weights, but if looking for a simple number, just start with 105 pounds for 5 feet, and add 5 pounds for each inch over 5 feet (in this case, 105 + 40). Young women often start slimmer and lighter than 145 pounds at 5' 8", but gain as the years go by, to end up over 145 pounds some years later.