This is gonna sound patronising but at 14 your body is still half child so you don't know how your body is going to develop. You're going through puberty so it's important to make sure your body has all the nutrition it needs. If you feel chubby, try and think about healthy eating (NOT restricting, just making sure your diet isn't total junk) and maybe finding a sport you enjoy and in time you will find a healthy weight (I'm willing to bet you already are at a healthy weight though).
The reason your diets haven't worked in the past is because diets don't work. They cause temporary weight loss but as soon as a person eats normally again the weight goes back on, sometimes even more than was there in the first place. Definitely don't use pills. Recipe for disaster.
If you're really concerned over your weight then speak to your GP, but honestly people come in all shapes and sizes and that's okay. Being skinny if your body isn't naturally that way won't look and feel as good as being healthy. Plus there are plenty of skinny girls out there who want some curves. Everyone wants what they can't have!