Adding content to the top of post’s based on their category

You can use the the_content filter for this:

add_filter( 'the_content', 'wpse420024_filter_content' );
/**
 * Conditionally adds content above the post content.
 *
 * @param  string $content The post content.
 * @return string          The filtered post content.
 */
function wpse420024_filter_content( $content ) {
    // Only run on the single post view.
    if ( is_single() ) {
        if ( has_tag( 'basketball' ) ) {
            $content="<div>My Additional Content</div>" . $content;
        }
    }
    return $content;
}

This code is untested. Try it out before putting it into production.

References

• the_content filter
• has_tag()
• is_single()