i am a lawyer in CT.  If you are not paying child support and there is an order for you to do so, then you may be subject to back child support.  However, if you have not yet been taken to court for child support there is no current obligation.  Nevertheless, if you are taken to court, a judge may impose child support that makes up for you not previously paying.  I suggest seeing a lawyer to help you in CA. If you cannot afford one, then speak to legal aid at the court house.