What's the logic behind simulating Price Fluctuation Graphs?

I am looking into programming a small and simple stock trading "simulator". My aim for the program is to learn how to program price fluctuations in stocks.

This will not be a real-time program, but a turn-based program.

The problem is I don't really know where to start regarding the actual fluctuations (or any types of 'random' graphs etc).

Will anyone give me a brief description of what my program needs to do to simulate this situation?