how to delete info off of one iphone that im not using

There's different ways and degrees of deleting data. One important thing to remember is to sign out of iCloud for any services you use before deleting the data on one device. If you do not then when you delete the data on one device this will be reflected on all other devices using iCloud with your Apple ID. If you have already one that then you will need to restore from a backup.

Change your iCloud settings - Apple Support

How to erase your iPhone, iPad, or iPod touch - Apple Support

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

For restoring read/watch these suport items -->

Click on this link to discover how to 'Restore your iPhone, iPad, or iPod touch from a backup' --> https://support.apple.com/HT204184

or watch this video if you're using a Mac to do this:

https://www.youtube.com/watch?v=Jbp3gGTQygg

Related information:

Use iCloud to transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch --> https://support.apple.com/HT210217

Some things are not stored in a backup. Click on this link and read "About missing information after you restore your iOS device with iCloud Backup" -->

https://support.apple.com/HT203768

If you login to iCloud and use the Find iPhone section you can select the device you want to delete and remotely erase the device. You should also be able to restore the iPhone 8 that was accidentally deleted from iCloud as well as long as it was set up previously.