Go to the doctor as soon as you can.
They'll give you a form to sign and probably refer you to a physio, they'll give you an incident report to fill out.
You'll also need to report the accident to the police as there was someone hurt and the insurance form needs a police report number.
I was rear ended at the end of feb and my neck didn't hurt too bad, went ot the doctor anyway. Lucky I did as I had some damage to my neck and shoulders, was all covered by their rego insurance.