Short answer: all of ‘em. Here is a biref description about what each field means and why it is important.
Category and Subcategory- get as close as you can with the vaguely-worded iTunes categories. It’s in your best interest for discoverability and to stay in iTunes.
Explicit/Clean: If you know your podcast will consistently be explicity, mark it as such. You can always override this for each individual episode. Note that if you have explicit content, and it isn’t marked, iTunes can yank your podcast out of iTunes. Do not taunt the dragon in its lair.
A homepage link:A website URL is required by iTunes. We know most producers have them but in case you don’t, we’re here to help— we can generate a URL for you if you don’t already have a homepage. However, having your own homepage allows you to post extras that may not quite fit in your episode, expands your reach and discoverability.
Owner Information: note that the email is public, so choose carefully.
Public Feed URL: if your podcast is already out in the world, this would be the link people follow to subscribe to it. If you are launching a new podcast you won't have one yet.
Enclosure Prefix URL: This is your Podtrac [https://analytics.podtrac.com/?gclid=EAIaIQobChMIi8O5ndnK4QIVBZ7ACh0z_g6eEAAYASAAEgIcgPD_BwE] or Blubrry [https://create.blubrry.com/resources/podcast-media-download-statistics/] prefix URL. This URL sits in your RSS Feed and makes sure that any downloads run through podtrac so that your metrics are recorded properly. PRX will help you create a podtrac account and prefix URL if you don't already have one.
Language: iTunes will need this for discovery. It should be the majority language your show is in.