Locate the openings in the case where you'll insert the test leads. Most multimeters have several jacks used for this purpose.
One is usually labeled "COM" or (-), which stands for for common. This is where the black test lead will be connected. It will be used for nearly every measurement taken.
The other jack or jacks should be labeled "V" (+) and the Omega symbol (an upside down horseshoe) for Volts and Ohms, respectively.
The + and - symbols represent the polarity of probes when set for and testing DC volts. If the test leads were installed as suggested, the red lead would be positive as compared to the black test lead. This is nice to know when the circuit under test isn't labeled + or -, as is usually the case.
Many meters have additional jacks that are required for current or high-voltage tests. It is equally important to have the test leads connected to the proper jacks as it is to have the selector switch range and test type (volts, amps, ohms) set. All must be correct. Consult the meter manual if you're unsure which jacks should be used.