There are many ways to understand the basics of realistic art, Youtube being one of them. But do you mean on a software or drawing (irl) because they both have their ups and downs like, software you can customise settings to your liking and to fit the art piece you plan to make, irl you have TOTAL control over what you draw - this can be more helpful if you're unfamiliar with software and can't use it to the full extent but have a good idea of what it is you're trying to create.