Just a few settings and compliance with simple rules of conduct are enough to significantly improve the security of a computer or smartphone:
Use account separation and strong passwords as described in Accounts & Passwords. This also applies to mobile devices: Set up a screen lock with PIN, pattern or other protection.
Lock your device when you are away to prevent others from accessing it. On Windows systems, for example, this works by pressing the “Windows Button” and the “L Button” simultaneously.
Activate or install a personal firewall on your computer. On Windows systems, the pre-installed firewall can be accessed under Control Panel -> System and Security -> Windows Firewall. For Macs under OS X, the firewall can be activated under Security in the System Preferences.
On Windows systems, enable User Account Control and set it to the highest or second highest level.
Enable automatic updates of the operating system and keep your other programs up to date.
Install a virus scanner on your device. You can read more about this in the Anti-virus Programs section.
Only connect removable media (e.g. USB sticks) of trustworthy origin to your device, as these can be infected with malware.