When you approach a casualty, it is difficult to remember which things to do first. It's important to do everything in the right order. The following flow chart shows you the right order to do things in - it's explained more fully on the next pages.
This video from the Red Cross is also really good, although it doesn't make it clear that you call for an ambulance before doing chest compressions. Also, you should now do 30 chest compressions- this video is quite old!