"Who are you, where are you, what do you want?" is the right advice.
On a busy frequency, you may well get a lot of information thrown at you in reply. The secret is to deal with it before replying. If I'm flying solo, I'll write down the relevant bits, then set the transponder, then reply. ATC know I'll be doing that, so they'll expect a few seconds delay.
You will learn when to expect a load of information, and as soon as you release the PTT, you'll grab your pen to write it all down. Then you read back the relevant bits of what you've written. If you've got it wrong, ATC will correct you. If you don't read it back, they'll ask you to. It's a good idea to know what must be read back! I work on "all numbers, except the wind, and clearances" which is approximately right.