Simple Moving Average Crossover

I downloaded Lispworks Personal Edition 6.1.1 and want to use it to create a simple program to trade stocks in the stock market. The details are as such : when the 15 day simple moving average crosses above the 30 day simple mving average crossover, buy the stock. When the 15 day simple moving average crosses under the 30 day simple moving average, sell the stock. How can i go about creating such a program? Thanks alot ...