how do I prepare for new phone device with my iphone6S

What other equipment do you have? If you have a Windows PC, you can create a backup with iTunes. Or with a Mac you can create a backup with Finder (or perhaps a slightly older version of MacOS with iTunes).

How to back up your iPhone, iPad, and iPod touch with your Mac - Apple Support

How to back up your iPhone, iPad, or iPod touch with iTunes on your PC - Apple Support

When you're ready, you can start the setup screen where it asks if you want to restore from a backup from a computer. It might be slightly different, as when you first turn on your new iPhone, it will ask how you want to set it up, and one option will be to copy over the contents of a backup. And unless you restore your new iPhone to factory condition, you'll only have one chance at this.

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

If you don't have a computer available (or with enough storage, there's now Quick Start, which allows two iPhones next to each other to transfer the data across Bluetooth. If you do that, make sure that you've got enough charge to do it, or preferably make sure that both are connected to power.

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

how do I prepare for new phone device on my old iphone6S

Presuming your new device is a new iPhone - Apple's support document Transfer data from your previous iOS or iPadOS device to your new iPhone, iPad, or iPod touch tells you all you need to know.

Use Quick Start and your iCloud backup as described.

According to Set up your iPhone, iPad, or iPod touch:

"Make sure that you back up your previous iOS device so that you can transfer the content to the new device."