I am 15 and 5'4" inches tall and my weight is 55 kg.But my stomach is bigger and much fatty that other body parts.How can I lose my stomach fat.

Exercise, exercise, exercise, and diet.

First off eating healthy is the key to any weight loss. This means fruits and vegetables!
As for the exercise, if it's you abdomen you're worried about there are plenty of exercises that are specific to this area and will help gain muscle in this area helping you to loose the weight and make your stomach tighter. A lot of exercise routines suggest sit-ups, but there are plenty of sit-up free ways to go about this.

Here are a few websites with exercise routines I'm also posting the link to a Jillian Michaels workout video that is specific for the abdominal muscles (I've done it and I was on the floor crying after day two- so it's pretty good) There are also tons of other free exercise videos on YouTube.

5 Ways to Lose Belly Fat (with Quiz) - wikiHow (http://www.wikihow.com/Lose-Belly-Fat)
3 Exercises Targeted to Burn Belly Fat / Fitness / Weight Loss (http://www.fitday.com/fitness-articles/fitness/weight-loss/3-exercises-targeted-to-burn-belly-fat.html#b)

And

https://www.youtube.com/watch?v=ZJ8Zdj0OPMI

Remember it won't happen overnight, but with a little time and work, you'll get there!!