Convert the particle's speed to meters per second if it isn't already in those units. For example, if you want to calculate the wavelength in meters of the note B-flat on a clarinet, but have its speed -- the speed of sound -- written as 1125 feet per second, you need to convert it, keeping in mind that 1 foot = 0.3048 meters: 1125 * 0.3048 = 342.9 m/s.
Represent the particle's frequency in hertz, keeping in mind that 1 MHz is 1,000,000 Hz. For example's sake, assume the B-flat note has a frequency of 550 Hz.
Divide the particle's speed in meters per second by its frequency in hertz to get its wavelength in meters. For the B-flat note coming out of the clarinet, this would be: 342.9/550 = .623 m.