First, take care of anyone who was hurt.  Then get the cars out of the road so no one else is hurt.  You don't have to "preserve the scene" until the cops get there.  If anyone was hurt, or there was more than $500 of damage done to the cars, call for police.  Usually a Highway Patrol Trooper shows up.  Cooperate with the police with a statement.  Statements made to the police are generally not admissible for the civil case.  Follow up with medical care.  If you are hurt, and it was not your fault, call an attorney.    We prefer you call us, but ANY attorney is better than no attorney.  All attorneys learn about personal injury in law school,  After talking to an attorney THEN you can start calling insurance companies, but not before.  You don't have to call your insurance company right away,  They can use your statement against you.  An attorney can't.