Backup your older iPhone
Assuming you’re coming from an older iPhone, you might want to backup the older model first. This is because if you use iCloud to sync your devices, you’ll want to make sure that the version that syncs with your brand new iPhone 12 has all the latest data like photos, contacts, messages, and more.
Go to Settings
Go to iCloud > Backup
Sync your old iPhone with your new one
If you’d rather save yourself the hassle of having to sync all your data and apps manually, Apple has a feature for its iPhones called Quick Start that basically transfers over most of the data from your old iPhone to the new one just by placing both phones right next to each other. All you need to do is have both devices turned on, and look out for the popup that asks if you want to transfer your old iPhone to your new iPhone.