What you need to do depends greatly on what’s full. Your storage allocation on your mail provider’s servers, or your iPad itself.
What’s the difference between local storage and iCloud storage?
If the mail server is full…
If your mail provider has limited your storage as most do, you’ll either need to pay the provider for more storage, or log into the mail provider’s web mail service and delete some messages from there.
If you’re using iCloud mail, you can use iCloud.com to do that mail message deletion, as well as relocating or deleting files stored in iCloud. Or subscribe to additional storage from Apple.
Tell us which provider you’re using for mail, and we may be able to add info.
If your iPad is full, you'll need to relocate some data, or delete it…
Manage your photo and video storage - Apple Support