(I intended to put a full Pico-8 starter tutorial here. Until I or someone else does, here are links to some resources to get started.)
Dylan Bennett's Game Development with Pico-8 is a zine. The first issue introduces the major topics and walks through some examples.
The Pico-8 Zine is a collection of articles and tutorials on Pico-8 subjects, written by multiple authors and edited by Arnaud DeBock. Many articles are intended for beginners.
Some people have made video tutorials and uploaded them to YouTube:
Books in production Edit
Tassilo Rau's How to Make Games With Pico-8 is a book in progress. You can pre-order the book and get early access to drafts.