I need a bra?!?! please help me?

im 13 and my mom thinks a cami is the same as a bras which it sorta is but im the only girl in my grade tht doesnt wear a bra. its really embaressing. im like a 36a wen i go to my frends house i wear her bras if i can keep it away from herr family and they fit good i dont just need a training bra i need a padded bra is there any way i can show my mom i need 1 i hav tried to tell her but she just says im flat.