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.