The wavelength of 100MHz in a radio signal is about 3 meters. You can get the 3 meters by taking 3 10? m/s and dividing by 10? HZ. To get the wavelength, you divide the phase speed by the wave's frequency. For more information, visit http://en.wikipedia.org/wiki/Wavelength#Sinusoidal_waves.
Adding a bit to AnneKay's correct answer: wavelength = speed of light / frequency, or (approx) 300,000,000 meters per second divided by 100,000,000 cycles per second (100MHz), or 300 / 100, or 3 meters. (the speed of light is used when dealing with radio signals)