Why would someone as thin as you are want to lose weight? Do you perhaps have an eating disorder lurking? If the problem is you don't like the way your body looks -- if you're quite thin but you still see skin that doesn't look tight -- that's an exercise problem, not a diet problem, though you may in fact be eating poorly -- no way for us to know what your diet is like. It may be, if you don't have a body image problem, that what you're really looking for is to look toned, and to do that you have to do resistance training.
To answer your question I have to ask a few of my own. 1.) How old are you? 2.) How tall are you? 3.) Are you born male or female?
All this matters. Say you are a ten year old boy who is 4'6" then it would be bad for you to weigh 100lbs, you'd be too heavy. You should weigh 70-80lbs then. What if you were a 20 year old female, 5'0", and weighed 100lbs then you'd be under weight, not healthy either.
Being male or female might not seem to matter though at first look but honestly it does due to biology. If you are a male your body fat should be 2-5% or more to live a healthy life. Women though are different, they must have at least 10-13% body fat. What happens when you get below these percentages though?! Well lets take a look.
"Body fat is important for a wide variety of body functions. Having too little body fat may increase your risk for heart disease, gastrointestinal problems and damage to your nervous system, Also, the University of Pennsylvania adds, you incur the risk of organ shrinkage. Because a very low body fat percentage can affect your immune system, you are more likely to suffer from infectious diseases as well. Limited fat stores may affect your body's ability to use insulin, making you more likely to develop diabetes."
"If you're a woman with a low body fat percentage, you may not be able to become pregnant, even if you have more than the amount of fat considered essential. You may need as much as 22 percent body fat to have regular menstrual cycles and become pregnant. Should your body fat decrease too much while you're pregnant, you may not be able to sustain the pregnancy."
"Being extremely thin can also cause problems with your metabolism, decrease your brain function and cause fatigue. Body fat helps keep you warm when it's cold, and without fat to help hold it up, your skin may sag and start to look wrinkled, making you look older than you are. If you don't have a lot of fat and aren't consuming enough calories, your body will start to break down muscle for energy, making you weaker overall. Eventually, if you don't start to eat and drink more to increase your body fat levels, you could become too dehydrated or starve to death, although starvation is uncommon in the United States."
I hope this answers your question