Measuring Air Temperature
Air temperature can easily be measured using a cheap thermometer such as the liquid-in-glass type, where the liquid (mercury or spirit) in the bulb expands and rises up a capillary tube (very thin) to a height that depends on the temperature, read in either degrees Celsius or degrees Fahrenheit.

Another type has a bimetallic strip that winds and unwinds as the temperature changes and makes a pointer move over a circular scale.

Where Should I Put it?
It should be put out of direct sunlight so that it reads the temperature of the air, not its own which has been heated by the sun. Ideally you need to put it in a screen, but if not then it should be hung on a north-facing wall.