First: try to warn oncoming traffic, if it can be done safely. Give reasonable aid to the injured. Call police and, if necessary, an ambulance. If property damage exceeds $700 -- which is very often the case -- you must notify law enforcement.
Then: call your insurer. They can start the claims process and talk you through the details.
Who was at fault? Insurance adjusters typically gather information from the drivers and passengers, any witnesses, and accident reports filed with the state patrol or local law enforcement. If fault isn't clear, adjusters may decide that the fault is shared between drivers.
Which auto body shop to go to? In Washington state, unless you signed a contract with an insurer to take your car only to a specified repair shop, you can choose where to take it. But the shop still needs to work with the insurer to agree on a price. If they don't, and the car's repaired, you may be responsible for costs not covered by the insurer.
What if my car was totaled? We get this question all the time, and have a lot of information available about how to determine the vehicle's value (be ready to negotiate), how to keep your damaged vehicle, etc.
Check if your policy -- or the at-fault driver's -- covers "diminished value." This is the difference between the value of an undamaged vehicle and what it's worth after repairs are made.
Rental car? If the other driver was at fault, his or her insurer will negotiate with you for rental car payment. If you were hit by an uninsured driver, your insurance may pay for a rental.
Finally, what's "subrogation?" Subrogation allows your insurer to recover its costs from the person legally responsible for the accident. In other words, they seek reimbursement from the at-fault person.