@slew Depends what you want to do.

The best way to get started with uxn is reading the uxn tutorial: https://compudanzas.net/uxn_tutorial_day_1.html

As well as the uxntal docs: https://wiki.xxiivv.com/site/uxntal.html

And the Varvara docs: https://wiki.xxiivv.com/site/varvara.html

For the OS, that's a whole different thing, also, it's very bleeding edge, you need to be familiar with the first link before diving into porporo.