How do I sync my iPad with my iPhone?

Assuming that both your iPad and iPhone are signed-in to iCloud with the same Apple Account (formerly known as your AppleID), to synchronise your devices via iCloud, you must ensure that you have correctly configured both device to use iCloud:

Settings > [Your Name / AppleID] > iCloud > [Saved to iCloud] See All - enable Apps and Apple Services that you wish to synchronise between devices

More information about using iCloud can be found within the iCloud User Guide:

iCloud User Guide - Apple Support

This external article may be helpful with understanding how to keep your devices synchronised:

https://www.lifewire.com/what-you-need-to-know-to-sync-iphone-to-ipad-1999222

Sync in what way?

You can have email sync, notes, messages, handoff, universal clipboard, etc...

What kind of sync are you looking for exactly? What re you trying to accomplish?

Start by looking at the following support document.

click here ➜ Continuity features and requirements for Apple devices - Apple Support