Programmer making a stock simulation web application

I hope this is the right place for this post.

I am developing a very very simple and basic stock simulation web application.

This is entirely simulated and fictional. When a trader buys or sells a share, it causes the price to go up or down accordingly. How is the increase/decrease in price calculated? How is it calculated in relation to the number of shares available, current price, current shares being traded, etc.?

Any help is very much appreciated! Thank you!