Here is what your html page should look like:
This is my heading
...and this is my sub-heading
Here is what your notepad should look like:
<!DOCTYPE html>
<html lang = "en-AU">
<head>
<title>My Page</title>
</head>
<body>
<h1>This is my heading</h1>
<h2>...and this is my sub-heading</h2>
You can have:
<h1>This is using the <h1> tag</h1>
<h2>This is using the <h2> tag</h2>
<h3>This is using the <h3> tag</h3>
<h4>This is using the <h4> tag</h4>
<h5>This is using the <h5> tag</h5>
<h6>This is using the <h6> tag</h6>