How you look at a given weight will depend on your body composition -- how much fat you have relative to lean mass. Two people can be different sizes even if they are the same weight and height -- the one with more muscle relative to fat will be leaner than the person with less muscle. Another factor is your frame -- some people have a smaller, more delicate bone structure than others, and therefore can still look healthy and fit at a lower weight.
To give you a range to start thinking about, use the BMI calculator on this site:
http://www.3fatchicks.com/diets/bmi-calculator.php. These numbers should not be taken as absolute gospel, but they will give you a range to start shooting for. As you lose fat (and, one hopes, gains more muscle through exercise), you can start narrowing down to a more specific goal. A LOT of people wind up happier at lower OR higher weights than their original goal -- it's a good idea to let your body itself guide you as you get closer.