You do not state whether you are a relative or close friend of the child or her parents. You also did not indicate whether you have a relationship with the child, i.e. former step parent. A legal guardianship in California can be a lengthy process and will require an investigation of you and the status of the child. There will also be an assessment of whether the child's parents approve of the placement and whether the parents will retain parental control over the child. Before you take this step I recommend that you meet with an experienced family law/probate attorney as well as a family counselor. If the circumstances warrant a guardianship, it may be worth the effort to take this rather significant step but you want to be fully prepared before you take any action.
This is not a substitute for a consultation with a family law/probate law attorney in your community.
Ms. Macklin is correct. Too much missing. What is your relationship to the minor? Is she living with her parents? If not with her parents, are her parents living? Is there currently a guardian.
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If you mean to take custody away from her parents, I don't think that you can without involvement of Child Protective Services.
The process involves filing a petition with the probate court and a court appointment.
Be careful what you wish for......