How can multiple airplanes prevent simultaneous communication between ATC?
As you fly more you will get to know the flow of radio transmissions and be able to identify when another exchange has completed. This comes with time and needs to be learned over time, it may be beneficial to listen into things like liveATC in your spare time to get comfortable. The FAA has a nice little brief on it here with some good pointers: 


  Listen before you transmit. Many times can get the information you want through ATIS or by monitoring the frequency. Except for a few
  situations where some frequency overlap occurs, if you hear someone
  else talking, attempting to transmit will be futile. You will probably
  jam ("step on") someone else's attempt to transmit, causing a need to
  repeat the call. If you have just changed frequencies, first pause and
  listen to make sure the frequency is clear.
  
  Think before keying your transmitter. Know what you want to say and, if it is lengthy, (e.g., a flight plan or IFR position report),
  jot it down so you do not waste transmission time trying to remember
  what you need to say.


A solid 1-3 second pause should ensure the channel is free for you to transmit. The nature of transmissions is to keep them short and to the point, even on a busy frequency you will have plenty of time get in what you need and get a response. 

As for the multiple transmissions at once, you will know! When two people attempt to transmit at the same time a nasty noise occurs. This is addressed here.