Summarize:

Plug a male-to-male 3.5 mm RCA cable into the headphone and microphone jacks on your computer. Plugging one end into headphone jack will cause any audio you play on your computer to go through the cable. Plugging the other end into the microphone jack will allow whatever you're playing on your computer to be broadcast to anyone you stream with. You can get the appropriate cable from most music stores or large stores like Walmart. Once you have your cable properly set up, sign into any streaming service that you would normally use to talk to your friends online. This could be something like Skype, Twitch, Ustream.tv. If you want to broadcast to more than one person, you'll need either the premium Skype subscription or to use a streaming service that allows for group chats. Choose the mp3 player you'd like to use to create and play your playlists for your station. You can use something like iTunes or even YouTube to play music. If you're interested in talk radio, you can record your own audio and then play it back directly from your recording software. Because you'll have a cable plugged in to the microphone and headphone jacks, anyone you stream with will hear the audio you play on your computer. Keep in mind that with your microphone jack occupied, though, you won't be able to add any live commentary without unplugging it each time. If you don't want to mess around with cables, you can use a subscription website that creates a radio station for you. Radioking.com, airtime.pro or looksomething.com only require that you pay for and log in to their services. You choose your audience size, the type of music you want to play, and the website does the rest.This is not necessarily the best option if you want to broadcast something other than music, because you can only choose the types of files that the website provides.
Plug a cable into your computer. Sign into an online streaming service. Play your audio. Use a radio station website.