Back up your old iPhone to save all your data
Use the iCloud:
Go to «Settings»
Log in to your Apple ID
Go to iCloud
Select the menu item «Backup»
Click on «Start back up»
You can backup to iCloud by only using Wi-Fi. The backup cannot be 1:1 size-wise in relation to iCloud.
The iCloud storage should be larger than the amount of used memory on the iPhone. For instance, if the iPhone has 50GB of storage, the iCloud should be around 45GB.
If there is not enough storage space, you are offered to buy more iCloud storage space. It's recommended to do this to simplify the data transfer procedure.