How to ask mum for 1st bra?

okay well my friend is FLAT chested like this line ______ and her mum braught her a bra shes 12 and she got a sixe 8a im a size 8-10a i no this cause when i was with my friends i got my bra sized i want to ask my mum for a bra but im scared i dont no what to say and if she says no it will be ummmm embarasiing hellp!!! . . . . . . . . . . . . . . . . . . . . .. . . I"M 13~~~~~~