The first thing you should do if you have been involved in an accident or come to the scene of an accident is to assess the situation so that you can prioritize what needs to be done first. Then, in whichever order is appropriate, you should warn, call 112 and perform first aid LABC.
LABC is an abbreviation that stands for: Life-threatening situation, Airways and breathing, Bleeding, Circulatory shock. It also tells you in which order to prioritize in the event of an accident. Life-threatening situation has the highest priority, Airways and breathing has the second has highest priority and so on.
It is the first five minutes after an accident that determines how great a chance a seriously injured person has to survive, so try to act as quickly as possible.