All you need is a computer runing Windows, Mac OSX or Linux and a broadband connection. To determinate how much upload speed you need, please see this article.
Advanced users could also use a mixer and add a microphone, maybe 2 computers one for the automation system running the music and the second computer to run the commercials.
Terrestrial FM Radio Stations could use a dedicated hardware solution such as a Barix Instreamer 100. Barix Instreamer 100 is a Standalone IP Audio Encoder Device. That way Barix act like an encoder, receiving the audio and send it to our servers without having a computer encoding the audio.
Play what you like to the world!