The most common security issue I see on various computers and phone is the use of weak passwords. I was thinking about writing something, but then I found this article: The Ultimate Guide to Password Security. You will find that people argue about the details, but this provides excellent advice. There is no perfect security, but if you follow this advice you will be better off.
If you spend enough time with it, it will help you understand the value (and dangers) of security questions and the value of 2FA (two-factor authentication).