Set the text-align property to center.

<html>
<head>
<style>
.centered {
text-align: center;
}
</style>
</head>
<body>
</head>
<body>
<h1 class="centered">Journey to the Center of the Earth</h1>
<p class="centered">Looking back to all that has occurred to me since that eventful day, I am scarcely able to believe in the reality of my adventures. They were truly so wonderful that even now I am bewildered when I think of them.</p>
</body>
</html>]]>

Make sure to pick text-align, followed by :, and finally center.