What else do I need to use a Raspberry Pi Pico?
Updated
You'll need a micro-USB cable to allow you to connect it to your computer or Raspberry Pi for programming. This is essential; without it, there will be no way to program the Raspberry Pi Pico board.
Optional items
A Raspberry Pi Pico header set allows you to use the Pico with a full size breadboard or a 400 Hole Solderless Breadboard for various projects.
Power
Want to power your project away from your computer once programmed? The Raspberry Pi universal power supply is perfect for this.
Books
There's also the excellent Get Started with MircoPython on Raspberry Pi Pico book for learning how to use the Pico with MicroPython.