Help with Custom Field and add_action

I need to create a TEXT field on edit post with the value of a meta

can anyone help me?!

thx =)

add_action("admin_init", "content_init");
function content_init() {
    add_meta_box("duracao", "Duração", "content_meta", "post", "normal", "high");
}

function content_meta(){
    global $post;
    $custom = get_post_custom($post->ID);
    $duracao = $custom["duracao"];
?>
	<div class="inside">
		<table class="form-table">
			<tr>
				<th><label for="duracao">Duração: </label></th>
				<td>
					<input type="text" name="duracao" value="<?php echo $duracao; ?>" />
				</td>
			</tr>
		</table>
	</div>
<?php } ?>