the simple solution would be to paste this snippet of code in your theme's functions.php file :
add_action('wp_head','keywords_and_desc');
function keywords_and_desc(){
global $post;
if (is_single()||is_page()){
if(get_post_meta($post->ID,'my_keywords',true) != '')
echo '<meta content="'.get_post_meta($post->ID,'my_keywords',true).'" name="keywords">';
if(get_post_meta($post->ID,'my_description',true) != '')
echo '<meta content="'.get_post_meta($post->ID,'my_description',true).'" name="description">';
}
}
And the just add keywords and description using the built in custom fields on a new/edit post/page screen, name the keywords field my_keywords
and the description field my_description
.
or you can use a very lightweight and simple plugin called Simple Meta Tags