how to change default icon of custom plugin?

Take a close look at add_menu_page hook, it provides argument to supply with icon url

<?php 
add_menu_page( 
    $page_title, 
    $menu_title, 
    $capability, 
    $menu_slug, 
    $function, 
    $icon_url, 
    $position 
); 

http://codex.wordpress.org/Function_Reference/add_menu_page

add_menu_page(
    __('Poll','menu-test'), 
    __('Poll','menu-test'), 
    'manage_options', 
    'manage-polls', 
    'poll_page',
    'plugins_folder Or Theme folder url/icon.png' 
);