According to the FDA, food is safe in the freezer when it is stored at a temperature of 0 degrees Fahrenheit. Keeping the temperature at or below 0 degrees in the freezer helps prevent freezer burn, which occurs when humid air enters the freezer.
Freezer burn is more of a concern with freezer appliances that do not have independent cooling systems. If the freezer has electronic controls, just press the minus indicator to lower the temperature. If the freezer has a dial control, it is usually preset at the factory to the appropriate temperature level. A freezer should be at least two-thirds full to operate at optimum efficiency.