To permanently delete your Telegram account:
Only delete your account if you no longer want to use Telegram. Deleting your account removes all your messages and contacts permanently. All groups and channels that you've created become orphaned and left without a creator but admins retain their rights.
Note: You may not be able to sign up again immediately with the same phone number.
If you need more help, please see this guide.
Related suggestion: Delete Account In-app