1. Call for help first.
2. Is there any danger to self in attempting to help the other person? It would not help the paramedics much if you add to their casualty risk....
3. Remove casualty from the danger area - if not, reduce the risk of further injury to the casualty - if there is a fire and you can safely put it out, do so. If you can remove him safely out of harm's way do so. If not, mark out the danger areas to warn oncoming road users to reduce the risk of further injury - flares , lights, etc.
4. Treat the casualty within the means available to you in terms of training / equipment.