Before I really ever did much of my own flying, I listened to ATC communications on my aircraft radio while watching the inbounds to Logan from my rooftop. Sounds goofy but I learned a lot about what and how to say it. I still turn on my transceiver occasionaly, just to listen and keep the lingo and rhythm in my head.
You don't hear the pros saying ah, um or waiting seconds to answer. No, they know what is expected and are ready with an appropriate reply. For example: United 123 you are cleared to 14000, not to exceed 250 knots. Roger 14 at 250, United 123. That is preferable to repeating the whole thing and wasting everyone's time.
Copying clearances can be tricky. However it is vital that they are copied verbatim as an error can be costly. So be prepared to copy complicated changes in routings, new clearances etc.
To make it easier to write as you fly, get a strap on writing pad arrangement for your thigh. They are commercially available. I even made my own when I was starting out.