All stock aircrafts have radios, if it`s not a stock aircraft it may not have a radio panel. You can open the radio panel with SHIFT + 2 in most planes, some may differ especially if it is a addon. We have chosen a Cessna 172 for demonstration, the radio-stack may look different from plane to plane, but it does the same job and has the same options. Once inside the airplane and having pushed SHIFT + 2, you will get something looking like this: (Solution continues under the picture)
Do notice the COM 1 that is now tuned to the frequency 118.75, 118.75 is the default frequency which all aircraft start with. Do also notice beside the frequency is a another frequency (127.90). This is called a Standby frequency. This is where you adjust the frequency, set it to the frequency you need to communicate with the active ATC, you can find the frequency by looking in the chatbox, GPS, Charts, Flight Sim Commander plus many more resources. Once you have entered the right frequency you push the button between the frequencies named STBY, that will transfer the frequency you just put in to be the active frequency, in the example here we have put in 118.10 (Solution continues under the picture)
Now you have the active frequency set, unless you have bounded up another button for it the standard button for transmitting with the radio is CAPS-LOCK. Hold CAPS-LOCK to speak. Do notice that SHIFT + CAPS-LOCK will make your radio transmit on all frequencies and this server has auto-kick on such event, do not use this function unless you have administration rights. It is also wise to monitor the frequency UNICOM 122.80 with your COM2 radio as you can listen in to two frequencies at the same time but just speak at one, notice the panel you switch the active COM with and also the button BOTH, that must be active for both the frequencies to work. The one chosen as COM1 is the active frequency you transmit on, press the COM2 button to make COM2 the active frequency for transmitting.
A good habit of getting into is setting your frequencies early. If a Ground and tower controller are online at the same point, set Ground frequency as your active and tower as your standby. That means with one button press, you will then be on the Tower frequency