The first step is to ensure you are actually working on the correct wiring.
Labels in the breaker box are not reliable (just my opinion) .
You may pick up stray current on your meter.
First check that the breaker is working. Multimeter from breaker connection to neutral.
I would ensure the breaker is off and do a continuity test for the wiring to the first box.
You can do this single handed by joining the wires at the box with a nut and testing at the breaker box.
A non-contact voltage sensor is a worthwhile investment. About $10 and will alert you to hot wiring without actually touching the bare wires.
If you are not comfortable with doing this stuff call an electrician.