But at 14 you need to gain weight, and unless yout already a fully formed adult (have the body of a 20 year old) you do still need to gain fat.
Fat is an important part of any women's body, seriously, yo get more as you age, its there to retain hormones, its there to retain lots of stuff (to keep you alive) if you don't have enough of it, your body stops producing certain hormones, one result is brittle bone disorder called Osteoperosis, another is having periods but not ovulationg and so not being able to have children, another is your hair falling out, your body growing hair to keep you warm if you don't have enough fat.
Are you talking about toning up? as your saying you don't want to lose weight but you do want to lose fat- so if you lose fat, your weight will drop, if you don't want your weight to drop, what are you going to replace the lost fat with?
Remember a healthy level of fat will vary from person to person but you really need to have enough when your young- your setting the foundations for the rest of your body's life here- you need good foundations or things will be difficult or nasty when your older.
Put it this way, I have a friend who is now in her 60s. When she was 15 she was very tiny, underweight, skinny had no body fat. She used to do alot of ballet. Only one time she slipped, fell, hurt (but not broke) part of her spine. Now luckily for her, the doctors worked out that as she was underweight and specificly had low body fat, she had malnourished bones. She was advised to gain weight, which she did begrudgingly, but her back problems remained until she was a healthy weight, way into her late 20s. She has since gone through the menopause, her periods have stopped, her bones began to decay as they do in everyone who hits the menopause. And you want to know the first thing that went wrong? her back! She had an x-ray, the problem is the exact same spot that had been damaged when she was 15. It just goes to show that old injuries when your at a low weight can stick around far longer then you think they can. And can come back to haunt you.
Please be careful- the last thing you want is to be confined to a wheelchair when your only 50 because you wanted a lowe body fat- by all means lower your body fat, but please make sure its within a safe range and still allows you enough body fat so your body can still grow up healthily and strong.