Phone for the police and ambulance since I don't have any medical training then yell out "Is anyone a doctor here?". I would stay with the victim until help arrived. Cover him/her with my coat if it's cold. It is important not to move the victim and cause further injuries.

Jura's answer is nearly perfect for a medical untrained person. Just try to protect the accident site, yourself and the victim from further harm if possible - as an example make some bystander wearing an orange jacket go down the street in the direction the traffic comes from. He should wave to coming cars to slow down. Important for the victim is that you talk to him - most severely injured people don't remember many things from their accident, but some can tell that there was soemeone who talked to them and they knew they weren't lost and alone. Your observation during caring for the wounded might be important if the victim loses his consciousness and you can tell the EMT or physician that arrives that the injured felt pain in the belly but could move arms and legs properly. Those observations could be very helpful. I am glad to see that you all try to help and call your local ambulance service ;-)

scream "man down"... "stay here, i'm going for help" now seriously, best i could do is call 112

Scream, yell "HELP" and call 120(if it happenes in China).

i know something first aid of course i help her/him. cowboy...