All you need is a computer running Windows, Mac OSX, or Linux and a broadband connection. To determine how much upload speed you need, please see this article.

What kind of internet connection do I need to be able to stream radio on my pc?

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 acts like an encoder, receiving the audio and send it to our servers without having a computer encoding the audio.