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

Just ask her, honestly, I don't think she'll say no. If anything, she probably just doesn't want to buy you one because she'd be admitting that you're growing up. My mom was like that, but she bought me one because, well, I needed it. Most girls start out with sports bras, not real underwire/cup bras because they don't really need all that support. So either ask your mom for one, buy one yourself, or ask your sister. You sister coul dmaybe bring it up to your mom or give you one of hers or an old sports bra. Maybe your sister could take you shopping for a bra? Either way, if you're startig to get boobs and are self conscious about it, its time to get one! Your mom isn't going to get mad at you for asking, maybe a little sad, but I"m sure she'll come through and get you one. Good luck!