One click on the "Documentation" tab provides links to several screencasts/tutorials. Are you looking for something specific? "Hello world" doesn't cleanly map to an environment as comprehensive as Pharo because there are so many options. Do you want "hello world" to output to the terminal (Stdio stdout << 'Hello world')? A window? A growl-style popup (UIManager default inform: 'Hello world')? The transcript (Transcript show: 'Hello world')?

Install it and try it. It might surprise you.

Books: https://books.pharo.org/

Pharo by Example should be a good starting point.

Developing in Smalltalk is unlike EVERYTHING else. As noted by seandenigris standard assumptions about things like "hello world" don't apply.