KevinAM has a good answer. Ammeters have a certain resistance, which can be quite high for milliammeters and microammeters, because the instrument itself measures volts (except for the early analog, moving coil meters). A shunt is needed to pass heavier currents, and reduce the resistance in the ammeter circuit.
To calculate the value of a given shunt, you need to know the voltage dropped across the ammeter, or it's resistance - same thing, really, as you know the current. A shunt will then have the same voltage across it, but at the required full scale current.
The purpose of an ammeter shunt is to provide parallel resistance to current that flows in to an ammeter. An ammeter can not handle a large amount of current in some cases, thus a shunt is used to distribute some current so that the ammeter does not malfunction when measuring big loads of current.