Should my 18 year old daughter get her own car insurance or should I put her on our car insurance we live in CA?

Unless you drive a luxury vehicle with comprehensive and collision coverage and your daughter is driving an old beater with just liability, it will likely be cheaper to add her and her car to your policy, assuming you live together. Contact your insurance agent for a quote on both scenarios to be sure. If your daughter is still a dependent on your taxes or you have an ownership interest in her car, be cautious as you still could be found liable for any damage she causes, regardless of if she has her own insurance or not.