Creating post custom field text area

Yes, you’ll want to create custom metaboxes for those. This is a pretty good intro into how to do that. But you might want to think through a few things:

  1. Are you sure you want that added to ALL posts? If not, perhaps create a custom post type and attach those fields to that post type instead.
  2. Maybe you really DO want it on Posts and not a Custom Post Type, but is it necessary for ALL posts? If not, you might want to add some custom javascript to conditionally add those fields if the user wants to do that kind of post. While this script is for page template, it could easily be applied to a post instead.
  3. I find it easiest to start with a Custom Meta Box library. WebDevStudios has a really great and regularly updated library to try out.

Good luck!