Please help me. How do i ask my mom to buy me...?

Just pull her aside and ASK HER. She will be glad you did and you will feel better too. Sometimes dealing with maturity subjects with our children is very akward for parents. We really dont want to embarrass our kids. However, she may feel like you would rather not have a bra. Tell her it would make you feel less self conscious. It will ease to tension for both of you.