File a petition to establish a parental relationship (form no. FL-200) and summons (FL-210) ASAP - do not delay, you should file right away. Obtain an attorney's assistance if you are able. The fact that the child may not be your biological child doesn't prevent you from establishing a parental relationship another way, such as holding the child out as your own and providing the child with support (see California Family Code 7611).
You can print copies of the necessary forms here: http://www.courts.ca.gov/forms.htm?filter=PA
Best of luck to you!
Answered on Sep 14th, 2015 at 1:21 PM