Call the police immediately and an ambulance if necessary.
Do not admit liability.
Record details of the accident including date, time, location, description of the accident, any injuries, etc.
Record details concerning the other party and vehicle, such as owner's name and address, phone number, vehicle year, make and model, plate number, and insurance policy number and company.
Obtain a copy of the police report from the attending officer, as this will have the accident report number for future reference.
These tips are only a few general rules of thumb, and may not fit all situations, when in doubt the more information gathered, the better.