Set the 'ServerName' directive globally to suppress this message

ok guys, I barely know what I am doing, so please be gentle and kind in any responses.

I've gone into terminal and started apache. I've set it up so it will be able to do multiple sites (using \user\myusername\sites.

it seems to be working as it will display the .php file I put as index.php

but for some reason I found the terminal command apachectl configtest So gosh, I figured I would run that command and see what happens (of course expecting everything to come back fine)! But here are the results I got:

AH00558: httpd: Could not reliably determine the server's fully qualified domain name, using MacBook-Pro.local. Set the 'ServerName' directive globally to suppress this message

Syntax OK

So I guess I need to set a global serverName somewhere???

Anyone know what I need to do?