How to get links to user and tag by its name?

Use the get_user_by and get_term_by functions:

// user
$user = get_user_by( 'login', 'someuser' );
if( $user ){
    echo get_author_posts_url( $user->ID );
}

// tag
$term = get_term_by( 'name', 'Sometag', 'post_tag' );
if( $term ){
    echo get_term_link( $term );
}