Songs cannot be sent as ordinary SMSes (text messages), not even broken down in to hundreds of them (think about how big the actual song file is, then bear in mind you can only get 160 characters per SMS).
You probably can't even send it as an MMS message either - the maximum size of an MMS message varies between handsets and between network operators (though <= 300kB is the norm).
For example, your phone might be able to handle a 2MB file (I don't use MP3 myself, so I'm not sure what size MP3 songs typically tend to be - I rip to WMA, and they tend to be 2MB+), but your network may limit the size of MMS messages to 100kB.
Equally, your handset and network may be fine with sending a 500kB file, but the recipient's network or handset may limit it to 250kB.
I think you're going about this the wrong way - you'll either need to use Bluetooth to send the file to the recipient (if they are in the vicinity), or to send it as an email attachment.
Obviously, for email, you'll need to either connect your iPhone to your computer at home and send it that way, or (if you've got email set up on your iPhone, and are happy to use a large chunk of your data allowance sending the song) send it from your iPhone.