I would like to make a respond to justify my reason why it would be the best interest for the court to do 50/50 custody for children
Divorce / Separation Lawyer
If the papers you were served with also ask for child support, you will need to fill out additional forms. Here is the link to access the appropriate forms: http://www.courts.ca.gov/1192.htm. Good luck!
Child Custody Lawyer
Responses are prepared on FL-320. If you need additional room for your explanation you can attach additional pages.
Family Law Attorney
To respond to a FL-300, you use FL-320. Para. 10 of the FL-300 is "other relief." You did not describe the other relief being sought, so I cannot suggest how you should respond to it. Where you respond to it is in paragraph 8 of the FL-320 form. Where you explain why 50/50 custody would be in your kids' best interest is in paragraph 9 of the FL-320 form (or, if you need more space, then in an attached declaration).
The foregoing is intended to provide general advice and does not substitute for consulting an attorney about your individual situation. My having answered your question has not created an attorney-client relationship between us.