You'll notice a neat little lock icon in the upper left of the URL bar, courtesy of Let's Encrypt, a non-profit organization that hands out SSL certificates for websites. You just have to install open-source software on the server, which proves you are the owner of a website, and you can automatically get a certificate.
I have been putting off installing it for a long time, because I was worried that it might break something. But it was super quick and easy to set up. Here's the guide I used (for Ubuntu 14, but worked on 18 just as well):