How do we send a song to friend through email?

First of all - before I say anything else - this is most likely illegal - it is file sharing and infringes on copyright laws!

You can attach files to e-mails using your e-mail client... generally the file should be smaller than 10MB to be sent to most e-mail servers. You can attach it, but it all depends on what e-mail client you are using. What do you use - webmail, Microsoft Outlook, or something different?