Push starting, also known as "bump starting" a car or motorcycle is a way of getting the vehicle started when either your starter motor or battery have given up the ghost. It consists of getting a vehicle into it's running state, pushing it to get some momentum, and engaging the transmission, to start the engine spinning. Be aware that to push start a car you need to have a manual transmission, and trying to push start an automatic vehicle will either fail to start it, or possibly damage the transmission. You should also avoid bump starting a vehicle with a catalytic converter as unburned fuel mixture in the exhaust may cause it harm. It can still be done in a pinch though. Though it may be tempting if you're alone and on a hill, it's a really bad idea to push the car down the hill then jump in to bump start. People have accidentally run themselves over trying this. It's best to find someone to help.
Step 1: Getting to a Running State
The first step in push starting a car is getting it in a state where it's ready to run. This usually just means disengaging the parking brake, having the key in the on or running position, and having the driver in place. This makes it so once the momentum of the pusher(s) is transfered to the engine it will be ready to use that momentum to start up and run.