Just about all mobile phones and tablet devices include ways to back them. This usually means connecting them to your work or home computer and following the instructions provided with the device, or following a procedure described by your phone supplier.
Some devices also provide backups over a wireless network, to "cloud" strorage provided by your service supplier.
There are too many possible devices avaiable in the marketplace to describe the steps you need to follow here, but you should bear in mind these points.
Back them up regularly
Get into the habit of backing up your device regularly, or frequently. If you lose your device, or if it's lost or stolen, you will appreciate having a recent backup as this will help you to get back to a working state, possibly with a new device or phone, much more quickly.
Configure them to back up automatically
Make it automatic. It is worthwhile taking time to learn how to configure it to back itself up regularly. If for example, you configure it to back up daily if it's connected to your computer, get into the habit of connecting it. The connection usually means that the device also charges itself, so if you habitually use the computer to recharge the device's battery, it will also mean you have an up to date backup.
Choosing to backup up to the service provider's cloud
Generally this is recommended.
However, there are exceptions. if you work with highly sensitive information at work, think carefully before you choose to back your device up to their cloud. You can be in breach of Data Protection law. Most mobile device supplier's "cloud" is actually located outside the European Economic Area, and allowing sensitive information to travel outside the UK into a foriegn power's jurisdiction can break UK Data Protection law.