Were there injuries?
This is first question and it is the most important! This IS the big picture. Most auto accidents for instance don't involve injuries, if you weren't hurt breath a sigh of relief, everything will be fine and a damaged car is really of no consequence in the big picture.
If you or other parties are injured, look after that first, get attention now! That's the priority.
What happened?
Provide a description of what happened. For a car accident explain weather, road and traffic conditions.
Where & When It Happened?
For a car accident, identify the road, area, direction traveled and time of day/night
Names and insurance info (if applicable) of others involved?
If police were involved, the report number and constables name?
Description of damage done?
If vehicle, what condition is it in now and where is it?
What do you need right now?
i.e.: taxi, rental car, lodging etc.