Making an Ammeter using Arduino Uno
One thing to bear in mind: 

240V IS DEADLY - USE EXTREME CAUTION

If you are thinking of directly sampling mains voltages, DON'T! (unless you know what you are doing, in which case why are you asking questions here?)

Sampling a range of 0-240V with a 12-bit ADC is not really ideal.  You get 0.23v per bit, and as you can usually count the LSB as noise, that is effectively about half a volt resolution.  Not really enough to sense anything meaningful from a small current shunt.  It'll be like "troll counting" - none, one, lots, many.