Find post id by a tag

How do I find an id?

Thanks but I know how to find an ID, what I am asking is:

If i give a tag to a post, how can I find this post id by querying its tag?

You can’t find one specific post but you can find all posts using a specific tag using WP_Query().

Ok how do I find the categories of this posts?

Basically I am using the plugin: https://www.ads-software.com/plugins/user-submitted-posts/

In the form you can insert a tag; this tag is attached to a post.

I am using the following and it works but if i duplicate the code below and I change the cat id in order to have all the tags within various categopries where the tags are used, it repeats the same tags for all categories.

<?php
		$cat=5;
		$yourcat = get_category($cat);
		if ($yourcat) {
			echo '<h3>' . $yourcat->name . '</h3>';
		}
		$args = array(
			'categories'=> '5'
		);
		$tags = get_category_tags($args);
		$content .= "<ul>";
		foreach ($tags as $tag) {
			$content .= "<li><a href=\"$tag->tag_link\">$tag->tag_name</a></li>";
		}
		$content .= "</ul>";
		echo $content;
	?>

Solution:

<ul>
                <?php
                    query_posts('category_name=Ambiente');
                    if (have_posts()) : while (have_posts()) : the_post();
                        if( get_the_tag_list() ){
                            echo $posttags = get_the_tag_list('<li>','</li><li>','</li>');
                        }
                    endwhile; endif;
                    wp_reset_query();
                ?>
</ul>