You can't just decide to become japanese.
So unless the details around your birth are amazingly mixed up(adopted-which your mother would know, switched at birth - which is rare, but possible etc), your mother would already know if you were Japanese, half-japanese or whatever.
My mum
Tell your mum how you feel about your dad.
ya mum
You don't have to tell your friend's mum anything
u tell her
anata no okaasan
Just ask her, she is your mum and she is there to support you.
when calling your mum mum, use okaa -Chan/-san/-sama  (お母 ちゃん・さん・さま) this can also be used 'haha'  (母) Sry for the lack of info. Still learning Japanese
"Mum you're a bad singer."
Just say mum I need a bra
You mean mom? ママ(mama)Mira
Hey mum, I have hair under my arms :O