I can't imagine there are many youtube videos about Torn, but you don't need them anyways. Most things are quite intuitive, there is a forum section for tutorials/guides/questions and can learn some basics from missions too.
The stock market mechanics seems straight forward, click the buy button to buy shares with the money you have on hand and sell button to sell the shares you have bought?
Any further questions feel free to message the friendliest guy on torn realistassassin. He's always here to help
how does this work