I appreciate your concerns about security! I am also still running a Mac with High Sierra. Even though you are on an older version of macOS you are still fairly secure as long as you are careful what you do online with your Mac.
One of the most important things you can do to maintain your security and privacy is to be careful what you wander through on the internet. The more risky sites you go to the more likely you could be the victim of a pfishing, trojan or other scam, so my #1 rule is always be careful about the sites you visit!
Second, always be careful with emails you receive, even if they "appear" to be from a known sender. Scammers are getting better & better at making their emails look legit. If it's in the least bit suspicious send it right to the trash without opening it. If you open an email don't click links, don't open attachments and don't reply unless you are absolutely certain the email is legit.
Here's a useful article from Boston University about protecting your web surfing: How to Identify and Protect Yourself from an Unsafe Website
And here's a article from PC World about The 17 Most Dangerous Places on the Web.
While there are no more security updates for High Sierra 10.13.6 there are updated releases for many apps you use, especially your browser, which I consider front line protection. If you are using Firefox (my preferred browser especially on older versions of macOS), you're in pretty good hands. Its basic security settings are good and you might consider adding Privacy Badger, HTTPS Everywhere and uBlock Origin to Firefox. All are free, the first 2 are from the EFF (Electronic Frontier Foundation), the last one from Mozilla itself.