function comment_validation_init() {
    if(is_single() && comments_open() ) { ?>        
    <script type="text/javascript" src="http://ajax.aspnetcdn.com/ajax/jquery.validate/1.9/jquery.validate.min.js"></script>
    <script type="text/javascript">
    jQuery(document).ready(function($) {
    $('#commentform').validate({

    rules: {
      author: {
        required: true,
        minlength: 2
      },

      email: {
        required: true,
        email: true
      },

      comment: {
        required: true,
        minlength: 20
      }
    },

    messages: {
      author: "Please fill the required field",
      email: "Please enter a valid email address.",
      comment: "Please fill the required field"
    },

    errorElement: "div",
    errorPlacement: function(error, element) {
      element.after(error);
    }

    });
    });
    </script>
    <?php
    }
}
add_action('wp_footer', 'comment_validation_init');