How can I format email notification after registration?

wp_mail from Codex

Use this example:

add_filter( 'wp_mail_content_type', 'set_html_content_type' );

wp_mail( $mails_to, 'The subject', '<p>The <em>HTML</em> message</p>' );

// Reset content-type to avoid conflicts -- http://core.trac.wordpress.org/ticket/23578
remove_filter( 'wp_mail_content_type', 'set_html_content_type' );

function set_html_content_type() {

    return 'text/html';
}