How can I calculate wavelength of radio waves?

If you know the Frequency $\nu$ (in Hertz) you can use the relationship:
$c = \lambda \cdot \nu$ where $\lambda$ is the wavelength (in meters) and $c$ is the speed of light (in vacuum is $3 \times {10}^{8} \frac{m}{s}$).
Rearranging you can write: $\lambda = \frac{c}{\nu}$