Assuming you are properly licensed to put this song/recording on your website.
Open iTunes. Go to your Library (Music). Find the recording you want to export. Drag it to your desktop. You will get a copy of your audio file that is encoded the same as it was in your iTunes Library (M4A if you use AAC when you import audio to iTunes). Upload that file to your website.
If you need the file to be in some other codec (say, MP3), you can do that conversion in iTunes also (before exporting the file). Change your iTunes > Preferences > General > Import Settings to MP3 Encoder. Then in the Library, select the recording you want to convert and do Advanced > Create MP3 Version. iTunes will create a duplicate copy of your recording but will encode it as MP3 instead of AAC. (When done, change the preference back to what it originally was, otherwise everything you import into iTunes will be MP3 instead of AAC.)
BTW, when you put the audio file on your website it does not necessarily become playable by everyone who comes to your website. It depends on a lot of different factors, many having to do with what the user's browser supports. You can overcome this by including a player on your website. The Wimpy MP3 player is good, and pretty popular.
