The primary source is Den danske ordbog: https://ordnet.dk/ddo/ordbog?select=I,4&query=I and https://ordnet.dk/ddo/ordbog?select=du,2&query=du . It can be a bit challenging for a beginner, but you should always try. It'll start working eventually.
Wiktionary is also often useful: https://en.wiktionary.org/wiki/I#Danish and https://en.wiktionary.org/wiki/du#Danish
Luckily, both of the sources agree: "du" is mostly the singular second person pronoun, so it refers to the single person one is talking to. "I" is mostly the plural second person pronoun, so it refers to several people one is talking to. "I" can also be used as a formal, polite term, but this usage is fairly rare in a Nordic context.
Note that English uses "you" as both a singular and plural pronoun. But many other languages do distinguish between addressing a single person and addressing several of them. Maybe you could find some resources for learning Danish from languages other than English, at least as a supplement? It would give a different perspective on the language. I think Clozemaster offers courses between Danish and French, at least, and probably German, too.
Example sentences from Danish to Dutch can be found on Tatoeba:
I don't know Dutch, so I don't know if these are helpful.