If your bra straps are leaving such significant marks in your skin, then FX is concerned that your bra is too tight. So the steps you would want to take would be to first find out your correct bra size and then purchase a bra that fits properly so that it will not leave those marks in your skin.
To learn about bra size and fit, how to properly measure yourself, and what styles of bras are available, please read our answer to At What Age Should Girls Start Wearing Bras? In addition to reading that answer, speak with your mother, other trusted female adult, and/or a saleswoman at a department store or lingerie specialty store who can assist you in sizing and choosing the best style/size/fit for you.