The voltmeter pictured is a "manual range" variety. It shows five DC scales and two AC scales. Each scale is intended to read values from zero volts to the maximum number of the range. In the case of the pictured item, the DC ranges are 0.2V (200 millivolts, mV), 2V (2000 mV), 20V, 200V, and 1,000V. The AC ranges are 200V and 750V.
To use this meter to measure a 12-volt system you would set it on the 20V DC range. To measure a 24V system you would need to use the 200V DC range. And to measure your 120-volt system you would use the 200V AC range.
All that said, if you are asking questions this basic, I would recommend you get a book on basic electrical concepts before you go much further. Home Depot and Lowes carry some books targeted to the amateur home electrician, and basic electrical texts are also available in the library.
You need to have a thorough understanding of the difference between DC and AC current, the importance of polarity, what grounding means, and the relationship between voltage, current, resistance, and power before you can really grasp how to use a multimeter such as the one pictured. Remember, electricity can kill you, or just let the magic smoke out of some very expensive parts.