I want to purchase a new iPad. How to do save my information and then delete everything on the old iPad?

To transfer your data, see this link:

Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support

To prepare the outgoing iPad for transfer, see this link:

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

These support pages should get you started with setup of your new iPad - and the methods of transferring your data from old to new iPad:

Set up your iPhone, iPad or iPod touch – Apple Support

Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support

Use Quick Start to transfer data to a new iPhone, iPad or iPod touch – Apple Support

If the donor iPad, from which you are transferring your data precedes iOS12.4, QuickStart will not be available.

“Transfer” is rather a misnomer. In reality, your data and setup is merely cloned to the new device. Nothing is moved or removed from the original device.

An alternative is to restore either an iCloud or iTunes backup to the new device:

How to back up your iPhone, iPad and iPod touch – Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

If you intend to trade-in, give-away or reuse your old iPad with a different AppleID, you would be well advised to prepare it properly for reuse - as this will ensure that the Activation Lock associated with the original AppleID is removed, along with all data and other sensitive information:

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