You have to conjugate the verb according the subject. The verbs dont have genus variation, so have no importance if you are girl or boy, when the subject is "eu", "I" in english, will always be "chamo-me" or "(Eu) me chamo". When the subject is "você", wich means "you" in english, the correct is "você se chama". So you have to say "Como (você) se chama?".
"Chamar-se" is something like "be called", for example:
"How he will be called?" (Como ela vai chamar-se?)
"He will be called Jorge" (Ele vai chamar-se Jorge)"