The good news is, you are not the only girl who is experiencing stretch marks.
During puberty, many girls get them on their breasts, on their hips and thighs and even on their buttocks. The marks are the result of you growing so fast your skin can't keep up with the changes. Your skin stretches to accommodate the increase in size.
The bad news is that there really isn't anything you can do to make the stretch marks go away. What will happen over time, is they will begin to fade and eventually you will hardly see them.
Massage lotion into the areas and just make sure they are covered by a training bra or a pair of shorts! They will sort out in the end I promise! Do some stretching exercises and keep your muscles in the area healthy - as they will help with the added weight of breast tissue and hips when they relly get growing! IF you haven't started your period yet, make sure you carry pads with you at ALL times so you can be ready for any surprises! It's a BIG change, but it's also a wonderful thing becoming a woman!
Big hugs sweety