I increase httpd.conf:

ServerAlias *.domain.com

Now I would like only one subdomain: h.domain.com to load hobbies.php

Which lines should i add into my .htaccess file to achieve this?

This will work, however i can't test drive it right now. Try it out.

SetEnvIfNoCase Referer "^http://h.domain.com" allow_hobbies=1

<FilesMatch "hobbies.php">
  Order Deny, Allow
  Allow from env=allow_hobbies 
</FilesMatch>

Note: You need to most likely not depend about this for security reasons, as you will find ways around it (relatively easy ones at this). The Referer header could be photoshopped.