Note that this method uses Font Awesome library from CDN.

In functions.php:

add_action( 'wp_enqueue_scripts', function() {
	if ( !is_admin() ) {
		wp_enqueue_style( 'font-awesome', 
		                  '//maxcdn.bootstrapcdn.com/font-awesome/4.3.0/css/font-awesome.min.css',
		                  null, 
		                  '4.3.0' );
	}
}, 99 );

Usage

In your HTML file:

<i class="fa fa-camera-retro"></i> fa-camera-retro

Will render this:

Another way of doing it without using <i class="fa"></i> is configuring it via CSS.

h4:before {
	color: #7d7373;
	content: "\f06a";
	display: inline-block;
	font-family: FontAwesome;
	font-size: 16px;
	padding-right: 10px;
}

Then in your HTML file:

<h4>fa-exclamation-circle</h4>

Will render this: