Why does the photoelectric effect depend on the frequency of the light?
The photoelectric effect is described by the following equation

$$E_\mathrm{max} = h\nu - \mathrm{WF_M}$$ 

where $E_\mathrm{max}$ is the maximum kinetic energy of the electron escaping from the metal surface, $\nu$ is the frequency of the incoming photon and $\mathrm{WF_M}$ is the workfunction for the particular metal. The kinetic energies of all electrons emitted are distributed from $0$ to $E_\mathrm{max}$. 


  Why was monochromatic light used in the experiment? Why is a proper
  frequency used and not any other frequency?


The experiment is typically performed by scanning through a continuous range of monochromatic wavelengths from lower to higher energy.  At some specific wavelength, the observer will notice that electrons start to be emitted (the threshold).  As the scan continues to wavelengths with even more energy, the emitted electrons will increase in kinetic energy.  From the threshold energy and the above equation, the experimenter can determine the workfunction of the metal.  If the experiment had been run with light containing many different wavelengths (non-monochromatic light), electrons would still be ejected, but you wouldn't know what the threshold wavelength was and you wouldn't be able to determine the workfunction of the metal being studied.

If you'd like more information on the photoelectric effect, here's a good, concise reference.