Resources

https://bestdelegate.com/. This website is specifically for high schooler Model UN. They have a ton of resources and guides for practically every aspect of MUN.

https://www.un.org/en/. The official United Nations website has some helpful information about the committees, recent developments, official documents, and more.  

https://www.cia.gov/the-world-factbook/. These are the CIA's overviews of every foreign country. It's the best way to get familiar with the basics of any country.

Your country's website will often explicitly state their stance on certain events and have a lot of other good facts too.

The APPMUN conference document (linked in this website) will have the topics listed along with a literal list of websites they recommend you use to research those topics. This is huge and you need to check out that list if you want to do well at a conference.