Summarize:

Many times, the voltage to be measured has a value that is unknown. For this reason, the highest range possible is selected so that the meter circuitry and movement will not be damaged by voltage greater than expected. If the meter were set to the 50 volt range and a common U.S. electrical outlet were to be tested, the 120 volts present could irreparably damage the meter. Start high and work downward to the lowest range that can be safely displayed. Insert the black probe in the "COM" or "-" jack. Next, insert the red probe in the "V" or "+" jack. There may be several Volt scales with different maximum values.  The range chosen by the selector knob determines which voltage scale to read. The maximum value scale should coincide with selector knob ranges. The voltage scales, unlike the Ohm scales, are linear. The scale is accurate anywhere along its length. It will of course be much easier accurately reading 24 volts on a 50 volt scale than on a 250 volt scale, where it might look like it is anywhere between 20 and 30 volts. In the US, you might expect 120 volts or even 240 volts. In other places, 240 or 380 volts might be expected.  Press the black probe into one of the straight slots. It should be possible to let go of the black probe, as the contacts behind the face of the outlet should grip the probe, much like it does when a plug is inserted. Insert the red probe into the other straight slot. The meter should indicate a voltage very close to 120 or 240 volts (depending on type outlet tested). Rotate the selector knob to the lowest range offered that is greater than the voltage indicated (120 or 240). The meter may indicate between 110 and as much as 125 volts this time. The range of the meter is important to obtain accurate measurements.  If the pointer did not move, it is likely that DC was chosen instead of AC. The AC and DC modes are not compatible. The correct mode must be set. If not set correctly, the user would mistakenly believe there was no voltage present, which could be a dangerous mistake. Be sure to try both modes if the pointer does not move. Set meter to AC volts mode, and try again. Whenever possible, try to connect at least one probe in such a way that it will not be required to hold both while making tests. Some meters have accessories that include alligator clips or other types of clamps that will assist doing this. Minimizing your contact with electrical circuits drastically reduces that chances of sustaining burns or injury.
Set the meter for the highest range provided for AC Volts. Insert your test probes. Locate the voltage scales. Test a common electrical outlet. Remove the probes. Reinsert the probes as previously. Try not to hold both.