Be a little suspicious and aware of what you get in your mailbox, popup windows, calls and texts. Are they a bit "off" or unexpected?
If a service provider offers it. It will greatly improve your security online.
No matter what operating system you are running, use antivirus.
Lock down your online accounts, be it Facebook, Instagram, Twitter, Apple or Google accounts.
Make good passwords. Easy for you to remember, the longer, the better.
You can use a passwordsafe and/or a manager to make things easier for you.
If your operating system offers encryption, use it. Make a backup before encrypting your system, and make sure to make a recovery USB key.
Buy an USB hard drive and backup your data. Do it regularly. Remember to backup your phone and tablet too.
When updates for operating system and programs become available, install them.