Being involved in a motor vehicle collision has to be one of the most unwelcome events a motorist can face. If this happens to yourself, there are certain things you should do (and not do), assuming that you are fortunate enough not to be hurt, hurt that bad, or stressed out enough to remember to take certain steps to memorialize the event in case of a legal claim arising later.
These steps should include the following:
Call an ambulance and/or 911 if someone appears injured or requests same.
Call the police or CHP as the case may be. Many police agencies will refuse to come to the scene if it appears to be a minor collision without visible injury to any of the participants, but don't be afraid to call anyway and don't be dissuaded from calling because the other involved motorist begs you not to (this usually means they don't have insurance or have something to hide, like an expired license or alcohol consumption).
If you have a cell phone camera or camera, take photographs and/or video of all visible damage to all of the vehicles, and also the place of rest of the respective vehicles assuming that they haven't been moved from the scene yet.
Share auto insurance information with the other driver(s). Take down the other driver's name, address, telephone number and if they will provide it, their driver's license number and date of birth. Write down the license plate number, year, and make of their vehicle as well.
Don't get into an argument with the other driver over whose fault it was, this can create liability situations for either or both of you that may eclipse who violated what section of the vehicle code. Be polite and don't be afraid to inquire into the other participant's physical well-being. If somebody asks you if you were hurt and you are not sure (some soft-tissue musculoskeletal injuries may not manifest symptoms for as long as 24-48 hours later) then say you are not sure or that you are "shook up", so long as that is true.
Don't be afraid to apologize, make amends, or express concern to the other affected participants. Offers to take care of the other motorist's damages at the scene are inadmissible as evidence under CA Evidence Code section 1152, and expressions of sympathy or benevolence for a person's injuries at an accident scene are inadmissible under CA Evidence Code section 1160.
If there are any non-involved witnesses who are kind enough to provide their names and contact information, then take that down in writing as well.
See a health care provider at the first symptom of injury or discomfort, as delays in seeking medical attention are seized on by insurance companies as an excuse to deny or compromise injury claims. If you have it, use your own medical insurance to pay for same, as the liable party is still responsible for the reasonable cost of your medical bills under the Collateral Source Rule. It is more important to see a doctor before you see an attorney.
Report the accident to your auto insurance carrier as soon as feasible, as reporting the accident is not synonymous with the making of a claim and you should find out what coverage is available to you (for property damage and medical bills) through your own policy before worrying about what the carrier for the other driver may or may not do. Your auto insurance carrier is mandated to provide you your agreed upon benefits under your policy regardless of who is at fault, as the other motorist's carrier won't pay a dime until they conclude a liability investigation and this may take weeks or even months to conclude.
Drive safely and make sure your automobile insurance is suited for your personal and financial needs.