In an emergency, what must I do till paramedics arrive?

Start CPR/follow 911. It is important that you remain calm and get help. Usually once the emergency medical services have been activated stay on the line with the 911 operator. Usually they can walk you through things to do. Best thing to do is to take a bls course (basic life support) through the red cross/american heart assiciation in your community. With this skill set you may save a life one day.
If you know CPR. If you know CPR or acls you will know what to do in a situation.If you do not know any of these it is better to call 911 and just give the victim some moral and general support. It is always better to know some basic skills to help people in need.