Stretch marks, whyy do i have them and how can i get rid of them?

hi so im 14 i weigh around 145 and im 5'7,,ive heard and stuff that 145 is a good weight for my height and stuff but i have strech marks on the inside of my thighs and im getting some on the inside of one calve,,,the ones in my thighs seem to have spread and are kinda dark,, i wanna know why i have them and how i could get rid of them?