How do I delete all of my emails on my iPad?

If you objective is to simply remove all email, associated Mailboxes and accounts from you iPad, just delete the Mail account(s) from settings:

Settings > Apps > Mail > Mail Accounts > [Account] > Delete Account

Should you have some other intent, if you provide more detail as precise what you need to delete, contributors will be able to provide a considerably more targeted response. Bulk deletion of mail messages is handled slightly differently for different types of Mailbox.

General information can be found here:

Delete emails on your iPhone or iPad - Apple Support