Editing Loop So It Targets Specific Tags?

Use the pre_get_posts action to modify the main query. Place this in your functions.php file:

function wpa71787_home_tag_query( $query ) {
    if ( $query->is_home() && $query->is_main_query() ) {
        $query->set( 'tag', 'reviews' );
    }
}
add_action( 'pre_get_posts', 'wpa71787_home_tag_query' );

You can set any valid parameters of WP_Query with this method.

Edit, secondary query excluding tagged posts, use tag ID:

$query = new WP_Query( array( 'tag__not_in' => array( 42 ) ) );
while( $query->have_posts() ):
    $query->the_post();
    // loop stuff
endwhile;