function theme_js() {
wp_enqueue_script( 'theme_js', get_stylesheet_directory_uri() . '/js/custom.js', array( 'jquery' ), '1.0', true );
}
add_action('wp_enqueue_scripts', 'theme_js');
In custom.js add the code like this:
jQuery(function($) {
$(document).ready(function(){
$('#sec1').addClass('shown');
$('#btn1').addClass('active');
});
});