I just got this game today and was confused how to respond to the radio calls both for the control towers and in multiplayer mode.
In the normal use of the game every time ATC calls your aircraft it will display options with numbers like
1=Please repeat
2=acknowledge
and so on depending upon the type of message ATC is sending you.
Multiplayer is complety different though depending upon how and what program you use to connect with.
I use a set of headphones and a mike and actually talk to the controller. But you can also type your responce to the message as well.
I woluld suggest you read through the learning center and take some of the lessons as they will help alot.
In multiplayer mode you will need to understand the terminology used by ATC and as a newbie unless you have some real world experience you will not know what they are telling you. 8-)