Exercism provides exercises and feedback but can be difficult to jump into for those learning Nim for the first time. These resources can help you get started:
Nim in Action is a great book, I would say that this is one of the best resources for learning Nim and what you can do with the language. I highly recommend this book as a good starting point.
The Standard Library Documentation is a great resource for looking at items that are part of the Nim's standard library and how to use them.
Learn X in Y minutes is good for getting a quick overview of the syntax of Nim and is good for a reference guide.
Dominik Picheta's Nim Playlist on Youtube is a list of videos going over Nim and solving some problems using Nim.
Also, the Nim community is extremely helpful and willing to help answer newcomer's questions, the Nim Community Page list all the places to chat with people in the Nim community.