Security is a big topic, here are a few things to get you started.
Encrypting Data in Transit
One of the most critical security features, one that is required for many modern APIs and progressive web apps is Secure HTTP also called HTTPS. A common misconception about HTTPS is that the only websites that need it are those that handle sensitive communications. If privacy and security weren't reason enough to protect your users, many new browser features such as service workers the Payment Request API require HTTPS.
Content Security Policy
Content Security Policy or CSP provides a rich set of directives that
enable granular control over the resources a page is allowed to load and
where they're loaded from.
Prevent Mixed Content
One of the more time-consuming tasks in implementing HTTPS is finding and
fixing content that mixes both HTTPS and HTTP. Fortunately there are tools
to help you help you with this.