Modeling Diffusion using Monte Carlo Simulations

Hello everyone, for a class project we need to model a caffeine molecule in air and how it diffuses as a random walk. The mean free path of the caffeine molecule at room temp. along with the pressure at 1atm is 20 nanometers. The mean free path is the average distance the the caffeine travels before hitting an air molecule. The average speed of the caffeine particle is 250 m/s between collisions. Using the Monte-Carlo simulations show that the average time it takes a caffeine molecule to diffuse by a distance d is proportional to d^2.

Here is the presented question. in all honesty i have no idea how to start other then i will be making a random number generator. Any help would be awesome thank you all!!