What to do when you are on a farm tractor how do you start it?

It depends on the make of the tractor. Modern tractors are usually started by the turning of an ignition key - as with a car. I once drove an old Ferguson tractor that had a brass button that had to be pushed in with my heel, while moving the gear lever into a slot marked start. Unless the brass button was pushed in, the gear lever couldn't enter the start slot!