How do I delete messages on my I phone and I pad at the same time.

If you are talking about deleting text messages on both devices at the same time, you would have to be syncing messages with iCloud. If you don’t sync messages with iCloud, you have to delete the texts on each device individually.

Use Messages in iCloud - Apple Support

Email messages or text/iMessages?

If you mean email, you must have the account configured as an IMAP or Exchange account. POP3 does not support synchronizing multiple clients.