Here's a circuit which might help you?, R1 and R2 form an attenuator, also known as a potential divider. You can work out the values from simple ohms law!.
But essentially, if R1 is 9 times as large as R2, then 90% of the voltage will be dropped across R1, leaving just 10% across R2 - so 120V in would give 12V out (divide by ten). However, you need to consider how much current your display requires?, and select the values accordingly. You will also need to make it adjustable somewhere, in order to calibrate the meter.