World Banknotes are a relatively inexpensive hobby and an educational way to discover the economic history, industry, and leaders of countries from around the world.
They represent art, history, and economics.
An easy way to begin is to purchase a starter package of notes from 25 or 50 different countries. These are easily available on eBay under a World Banknote Set search. Be sure to buy uncirculated banknotes from many different countries.
World banknotes are identified by their country and Pick number.
Most people prefer to collect notes in uncirculated condition as they are more desirable and retain their value.
How to collect banknotes
Here is a good booklet on Banknote collecting. https://www.theibns.org/intro-to-banknotes/Introduction-To-Banknotes-And-Banknote-Collecting.pdf
Roberts World Money has a nice series on beginning a world banknote collection.
An Introduction to Bank Note Collecting
A Collectors Perspective on World Banknotes