No need to apologise for any lack of knowledge. Postfix mail log is called mail.log and hopefully is in /var/log/ along with a whole host of other log files.

When you set to PHP and hit send, do you get the mail sent message in admin? If so, thats a very good sign, and don't worry about sendmail, etc at this stage, not important.

When you set the mail transport to PHP, it uses PHP code in PHPMailer to send the mail from the Apache server, usually as a mythical user called www-data, who is the Apache servers phantom persona to do such things. The question at present is whether Postfix is prepared to accept mail from this ethereal fellow.