you can just make a file in your root named robots.txt if you don’t already have one
User-agent: *
Disallow: /myPage
is the format for that one page
there is no allow, only disallow. disllowing pages you follow the above format
WP may not have generated a robots.txt, I believe it only would have if you wanted your site not indexed
I’m not sure if WP has inbuilt mechanism for not indexing your admin stuff….but mine’s never been indexed. If you are worried, to block entire directories you add
Disallow: /wp-admin/
it’s one entry per line/disallow