http://www.php.net/manual/en/function.mail.php
should get you started 🙂
just note that sending (large) attachments can sometimes be a problem -- so i've read.
basically, you will want to give the <input type=file> box, make sure they enter a file, then when they click on submit, you will receive the file right along with all other form variables (name, email, etc).. and proceed accordingly (validate, send email, etc)
cheers,
kyle