<?php echo "Hello World!"; ?>
A tip: you're better off using single quotes for echoing just text/strings, e.g, echo 'Hello world!'; since it is slightly faster. Using double quotes makes it slower, because it searches for variables and other stuff that can be used in PHP with double quotes, that can't be used with single quotes. Otherwise, good tutorial!
Dude explain more about php like what it stands for, what does the <?php and ?> do and what does echo do. also make it more like a step by step format to make it easier to read