You can’t get rid of stretch marks. They’re scars. They’re permanent. They’ll fade over time, but they won’t ever go away completely.
And they’re not a big deal. Almost everyone has them. I’m assuming you meant ‘not a fat person’ or something along those lines, but seriously skinny girls also get stretch marks. They happen when you grow a little faster than your skin does. Guys get them when they grow, too. They’re a part of life.
Just learn to accept them. They’re part of your body. There are scar-fading things you can use if you want to lessen their appearance (I don’t know what they are off the top of my head so don’t ask me - just go Google it) but they’ll fade naturally anyway as time passes.