How to get the first tag of a post as a hyperlink?

Instead of

echo $tag->name . ' ';

use

echo get_tag_link( $tag->term_id );

See the Codex on get_tag_link() and term_id.

And encapsulate the code in a function. Put this into your functions.php:

function wpse_49056_first_post_tag_link()
{
    if ( $posttags = get_the_tags() ) 
    {
        $tag = current( $posttags );
        printf(
            '<a href="https://wordpress.stackexchange.com/questions/49056/%1$s" itemprop="url"><span itemprop="title">%2$s</span></a>',
            get_tag_link( $tag->term_id ),
            esc_html( $tag->name )
         );
    }
}

In your single.php you just call the function where you need the link:

<?php wpse_49056_first_post_tag_link(); ?>