Add a custom widget as wordpress plugin.

my-test-widget.php

<?php
/*
Plugin Name: My Test Widget
Plugin URI: http://wordpress.org/plugins/???
Description: Test widget.
Author: Jeonghwan Kim
Version: 1.0
Author URI: mailto:ej88ej@gmail.com
*/

class MyTestWidget extends WP_Widget {
	public function __construct() {
		$widget_ops = array (
			"classname" => "my_test_widget",
			"discription" => __( "My Test Widget" ),
			);
		parent::__construct( "my_test_widget", "My Test Widget", $widget_ops );
	}

	public function form( $instance ) {
		$defaults = array('title'=> 'My Notice', 'name'=> '', 'notice' => '');
		$instance = wp_parse_args((array)$instance, $defaults);
		$title = strip_tags($instance['title']);
		$name = strip_tags($instance['name']);
		$notice = strip_tags($instance['notice']);
		?>
		<p><?php _e('Title')?>:
		<input class="widefat" name="<?php echo $this->get_field_name('title');?>" 
		type="text" value="<?php echo esc_attr($title)?>" /></p>
		<p><?php _e('Notice')?>:
		<textarea class="widefat" name="<?php echo $this->get_field_name('notice');?>">
		<?php echo esc_attr($notice);?></textarea></p>

		<?php 
		}

	public function update($new_instance, $old_instance){
		$instance = $old_instance;
		$instance['title'] = strip_tags($new_instance['title']);
		$instance['notice'] = strip_tags($new_instance['notice']); 
		return $instance;
	}

	public function widget( $args, $instance ) {
		extract($args);
		
		echo $before_widget;
		echo $before_title . $instance['title'] . $after_title;
		echo '<ul><li><a href="">' . $instance['notice'] . '</a></lu></ul>';
		echo $after_widget;

	}
}

add_action("widgets_init", "my_register_widgets");
function my_register_widgets() {
	register_widget("MyTestWidget");
}
?>