We will need a MicroSD card to hold the Operating System. (OS) I suggest a 32GB class 3, but a class 1 will work.
The last version released was Buster, the most recent is Bullseye in both 32 bit and 64 bit.
In any case you will need to use a flash program to copy it to the MicroSD card.
I used BalenaEtcher https://www.balena.io/etcher/
Using either program flash your chosen OS to the MicroSD Card.