You access the radio interface through your inventory, the same way you access the Notepad.
You can "tune" the radio to frequencies, frequencies range from 87.5 to 108.0. In the interface you can click the knobs at the top left and right to change the frequency manually, or you can click the middle button that displays the frequency to enter a custom value. Sometimes it can be problematic due to the way floating point values are interpreted by the server, but it's not common.
Frequency 108.0 is the Global Frequency, if you talk on this then your chat will be broadcasted to everyone. On any other frequency your chat is only broadcast to players on the same frequency. You can quickly switch to global either by clicking the button in the bottom left or by entering the "/g" command.
Global chat is also possible by typing "/g [text]" which allows you to send a global message without actually changing frequency.
Local chat is possible by turning the radio off, this can be done by accessing the interface and clicking the button in the bottom right or by quickly typing the command: "/l".
Similar to the quick global chat, you can type "/l [text]" to send local chat while maintaining the frequency you are on.