Kylie Jenner wears wigs. She has damaged her hair so much by dyeing and styling it with heat. Stay away from excessively dyeing/heating your hair. Kylie herself admits that her hair is extremely damaged! On SnapChat, Kylie said: "The top of my hair, is like, kind of destroyed from my bleach, so I'm, like, self-concious about it."
When you use a straightener, you are breaking the hydrogen bonds within your hair. The bonds are fixed once H2O is added, but the hair looses its vitality, shine, and softness. making chemical changes to the structure of your hair is never a good idea. Instead, try de-frizzing products for some beachy waves. You can also blow-dry your hair with a round brush to get a controlled and sleek look. It is fine to straighten your hair once in a while, for a school dance, an awards ceremony, graduation, prom, but do not make it a daily routine. When you do straighten your hair, use a straightener with ceramic, floating, plates instead of metal ones. The Kardashian straightener is no better than any other ceramic-plated straightener, it is just endorsed by them. They are paid to put their faces on it because companies know that they will see in increase in revenue.
My advice is this: I know you don't want to hear this, but it is better to have soft, bouncy, curly hair than it is to have straight, wiry, brittle, crispy, and weak hair.
You are only 15 years old, if you start heating your hair regularly at such an early age, I can guarantee that your hair will be incredibly damaged by adulthood. My mother only heated her hair for 10 years, but now, her hair became wiry like straw. No matter what she does, it will never return to its healthy state. To tame it, she has to heat it further, which only damages it more. The bottom line is: it is better to have beachy, curly hair than it is to look like a scarecrow. It might not happen immeadiately, but after 10 years your hair will be damaged beyond repair.
I've seen the damage it's done to my mother's hair.