How to measure voltage AC using multi-meter
First, make sure you multimeter is set to measuring AC voltage. If your meter has multiple AC voltage range (e.g. 1000, 100V, 10V), use the largest one.

Make sure the black lead goes into the "COM" jack and the red lead into the the jack with a letter "V" (or something like "VΩ", but not "A" or "mA").

When measuring, remember voltage is always measured between the black wire and the red wire. So in your earlier question, you could measure something like "the voltage between (1) and (2)" by touching (1) with red and touching (2) with black. It does not make sense to say "the voltage of (1)" without mentioning a reference. 

Of course you can measure the voltage between a point in a circuit and the ground, by sticking the black wire into the ground wire ("earth cable"). But if the circuit itself is not grounded, or the "earth cable" is not actually connected to the ground, that voltage doesn't mean anything.