If you are a woman then you must say "obrigada" and if you are a man you must say "obrigado".

Obrigado for boy and men Obrigada for girl and woman

If you are a girl you have to say: obrigadA , if you are a boy, you have to say obrigadO.

Obrigado is used by men. Obrigada is used by women. And women also say obrigado more frequently than obrigada.

"Obrigado" we use for men, and "Obrigada" we use for woman.