The most important action anyone can take in the first seconds and minutes after a crash is to ensure that you as an individual are as safe as you can be. Get yourself to a position of safety before taking any other action.
Once you have made yourself safe, make the scene safe if it does not put you at risk. Make on coming traffic aware of an incident using cones or signs if they are available (some car users have these in their boots).
If there are casualities assess the severity and help those who need it if you are able, ring 999 and perform simple first aid if necessary and you are able to safely do so.
Learning simple first aid through a formal first aid course can save lives in situations like these. Three top tips are:
1) to put pressure on a catastrophic bleed using a large wad of material.
2) to tilt an unconscious persons head from a hanging down position to an erect position
3) simply talking to and reassuring a casualty can make significant difference