Determine the speed of the wave in meters per second. The speed of sound varies depending on air pressure, humidity, temperature and location above or below sea level. The speed of sound in air is 340 meters per second; the speed of sound in water is 1,500 meters per second. You can set up two microphones in a line moving away from the sound's origin and use a timer to measure how long it takes for the sound to travel between microphones --- measure the distance between microphones and divide it by the travel time.
Determine the frequency in cycles per second using a frequency counter. Frequency is often measured in Hertz --- 100 Hz means 100 waves are created in one second.
Divide the speed by the frequency to determine the wavelength --- your answer will be in meters.