Implement External API into WordPress [closed]

First, I highly recommend you get to know your data model before doing anything. For this purpose I recommend dumping the raw JSON into a linter or formatter (e.g. JSONLint) to see what you’re dealing with.

That said, what Brian suggests is appropriate — you should take a look at the object you produced from json_decode by passing it through print_r. Without knowing exactly what you’re up to, it’s tough to make any specific recommendations, but since most of the data appears to be embedded within the standing property, I’ll give you a contrived example of how you could work through those, rendering a <ul>...</ul> for each standing and an accompanying league caption title:

<h1><?php echo esc_html($fixtures->leagueCaption); ?></h1>
<?php
foreach($fixtures->standing as $standing) {
    ?>
<ul>
    <li><strong>Position:</strong> <?php echo $standing->position; ?></li>
    <li><strong>Team:</strong> <?php echo $standing->position; ?></li>
    <li><strong>Games Played:</strong> <?php echo $standing->playedGames; ?></li>
    <li><strong>Points:</strong> <?php echo $standing->points; ?></li>
    <li><strong>Goals:</strong> <?php echo $standing->goals; ?></li>
    <li><strong>Goals (Against):</strong> <?php echo $standing->goalsAgainst; ?></li>
    <li><strong>Goals (Difference):</strong> <?php echo $standing->goalsDifference; ?></li>
</ul>
    <?php
}
?>