How to place script in footer?

If you want to output a single line of javascript, you might not need to put it in a js file and go through enqueuing it and stuff. Simply output it by using the wp_footer() action hook:

add_action('wp_footer','print_my_script');
function print_my_script(){
    echo '<script> // Your script here </script>';
}

However, this is good just for small scripts. For larger script and js files, use wp_enqueue_script() instead.