When you have your MP3 file in a playlist in iTunes, or whichever player you use to listen to music on your computer, you can click on the song title in question and drag it onto your desktop, where it will appear as an MP3 icon. If you wish to rename the song or add any further details such as numbering it, or adding a date, you can do it now. Simply click on the file name as it appears on the icon, and you'll be able to type in new details to replace the old file name.
Open up your email program and compose your email message. Then, click on the "Attach" button so that you can browse for the music file you wish to attach and send. Selecting "Desktop" from the resulting drop-down menu will display all files stored there. Simply click on the song title to be attached.
The time taken for this attachment process depends upon the speed of your computer and the size of the file, but it generally only takes a minute or so. When the file is attached, enter the email address of the desired recipient and then hit the "Send" button. When the recipient opens the email, he can download this attachment to his own hard drive and play the song file in the music player of choice.
Check your email program to see what your file size allowance is. If your MP3s are at a high bitrate, they may be too large to email, and you may want to consider changing the bitrate you choose when you create your MP3 files. Using a lower bitrate may result in a slightly inferior audio playback quality, but it will keep the file size down to a manageable size so that you can email your songs.
Unsolicited attachments often end up being snagged by spam filters, so it's a good idea to let people know in advance if you're going to be sending them email attachments.