There are plenty of great free resources, and the official one should always be something you're at least somewhat familiar with:

https://www.faa.gov/air_traffic/publications/atpubs/aim_html...

But if you're willing to spend a little money for some organized material, this is a good reference:

https://pilotworkshop.com/products/vfr-communication/

If you're not ready for VATSIM:

https://arsim.ai/

A good progression could be something like:

  - study FAR/AIM and the above Pilot Workshops books for some specific scenarios
  - sit in a chair and pretend you're doing radio comms for those same scenarios, and just practice saying out loud your parts as if you were really in that situation
  - run through the scenarios with ARSim (this helps you get used to parsing the incoming info)
  - https://www.liveatc.net/ - listen to real world conversations, and try to anticipate what info will be given and what responses will be expected
  - VATSIM and/or https://www.pilotedge.net/
  - get up in the air and talk to real ATC

If you do the above steps, by the time you get to the last one, you might fumble it a little at first but otherwise you'll do ok, and you'll be head and shoulders over many, many other pilots out there.

arsim is definitely one to keep an eye on, it's fairly new but a great example of what generative AI might be good at (given the limited number of words used during ATC interaction)