Make sure you speak clearly over the radio. If you try and talk to fast, it is more difficult to hear you and it winds up taking more time because you have to repeat yourself. The controller wants you to speak clearly and loudly, don’t mumble. Receive the instruction once, understand the instruction, communicate clearly and loudly your unambiguous response.
The phonetic alphabet and numbers use is a guide on how to speak clearly and unambiguously. Using these ways of saying letters and numbers, nobody will ever be confused as to what you’re trying to convey. Saying the same thing in a different way involves the listener trying to interpret the sounds you’re making into these things he is familiar with. This can work just fine under many circumstances, such as when the speaker and listener speak the same primary language, similar dialects, are from the same or close geographic regions, or are of similar backgrounds – so that they receive a message in a similar way. The more different the speaker and listener, the more important it is to use a standard way of speaking.
Time, speed, direction, and frequencies are stated by saying each number individually. Altitudes are also spoken as individual numbers, except it is okay to say "hundred" or "thousand" instead of spelling out multiple zeros. At and above 18,000 MSL, the altitude is stated as its flight level. "Flight level one eight zero", for example.
When you check on frequency, take a moment to listen to the frequency. It’s not a race to see who can be the first one to check in with the new controller. Wait for some dead air and make your check on. It is also polite to not wait too long to check on, as well. The controller is mentally thinking about you and waiting for your check in, even when he is busy. The best check in is polite and patient, but also timely. Don’t take so long that the controller has to start wondering where you are. If you take too long, you will really increase his workload because he’ll have to start making calls to other controllers to look for you. This increases the workload of several controllers. So, you don’t want to waste any time checking in, but don’t be rude about it.
Once you’re on frequency, listen to what’s going on. While it’s not going to be possible to form a very clear picture of what the controller is doing, you can gain a general level of awareness, which can be quite helpful. It’s kind of like listening to gossip. Is everybody talking about thunderstorms today? Icing? Turbulence? Unforecast weather? Something special like active military airspace or presidential movement? Is another guy in a similar type airplane out ahead of you dealing with ATC? Chances are the stuff that is concerning everybody else is going to concern you too. You can gain a heads up to things you might think about.
When receiving airborne clearances, an altitude or airspeed assignment, for example, you are not required to read back the clearance unless specifically requested to do so. However, it is good practice to read everything back to the controller. It provides you and the controller both with assurance you heard the clearance correctly. Also, the controller might have accidentally said the wrong thing, such as assigning the altitude he is protecting instead of the altitude he wants to give you. Your read back is the last change for you both to catch a mistake in the clearance. If air traffic control issues a speed, you are responsible to maintain that speed within 10 knots or 0.02 mach.
Finally, there exists a magic word: unable. You can and should use this word anytime air traffic control requests you to do something you are not comfortable with. Air traffic controllers always have multiple plans in mind for the control of your flight. If you unable a clearance, the air traffic controller will simply go to plan B. If compliance with a clearance would violate a regulation, you should immediately inform ATC you are unable to accept the clearance. If you have to deviate from a clearance for any reason, you must advise ATC as soon as possible of the deviation.
Reference: AIM 4-2-7: Phonetic Alphabet
Say 1945Z as "One niner four five zulu".
Say 150 knots as "one five zero knots".
Say 200 degrees as "Two zero zero degrees".
Say Heading 45 as "Heading zero four five".
Say 132.02 as "One three two point zero two".
Say 2,100 as "Two thousand one hundred".
Say 10,000 as "One zero thousand".
Say 23,100 as "Flight level two three one".