Apart from CAP413, there's a GA Supplement document, which I found more useful when learning.
It can be difficult when starting on the radio, but we all were beginners once. I still cringe when I remember on my 1st or second lesson as a student asking ATC to "Say again" when he was signing off with "Squawk 7000". I couldn't hear him and didn't know what to expect, but I could hear he was very annoyed....and my instructor embarrassed.
Try to manage the workload so you're not talking when you're busy with something else, and don't be shy to say "Standby" if you're busy flying and can't process the communications at that time. I also find it easier to wait a few seconds until I'm over a landmark before calling my position, for example.