Protecting your physical devices is just as important as protecting your online identity, and they are often linked.
Make sure your devices are password, PIN, or biometric protected.
Never share your login details with another person.
If you use a shared device, set up different users/personas for each approved user.
Make sure you secure your devices in a locked space when not in your possession.
Never provide remote access to your device unless you are 100% that the person who wants access is legitimate.
Only download software from legitimate sources and closely scrutinise reviews (are there enough of them? are they all too recent? do they appear suspicious).
Most anti-virus programs will check apps for malicious content before they can infect your device.