How to display all posts assigned to some tag?

I would recommend giving the wordpress tag template page a read but the basic idea of what you want to do is something like this:

Create a tag.php fileand place it in your template. This file will act like any other wordpress template and allow you to use “The Loop” which will allow you to cycle through each post that matches the url and display the info however you’d like.

<?php 
    if ( have_posts() ){
        while ( have_posts() ) {
            the_title();  //Displays the post title
            the_excerpt(); //Displays the post excerpt
        } 
    } 
?>

That block, inside tag.php will do what you are basically looking for in an extremely basic way. Add HTML wrappers and/or call different functions to achieve the look you are going for.