Technology stack/best practices for building a freemium website?

I've built a purely client side JavaScript website with Angular that uses Google's Oauth2.0 to let users login and retrieves data from an API and presents it in a way I think users will find valuable. I'm really proud of it, and Treehouse was instrumental in figuring a lot of it out.

When I built it, I wasn't even considering making it a paid service... but now I'd like to do just that.

I know I probably need to take this server side to hide my business logic (I've been looking at the Node courses on here). I also know I probably need a database to keep track of whether the user has paid (I've been looking at Firebase) But I'm having a hard time figuring out the best way to piece these technologies together and if there are other things out there that might help me (Stripe?). I'm also interested in best practices for the sake of security for my users and so I don't accidentally build some anti-pattern.

I know these are very broad questions. I'm not looking for any specific answers necessarily. More just direction as far as what I should be investigating to facilitate my process. Even just short list of things I should Google search would be helpful.

I've never really done anything server side like this, so I'm feeling a bit lost as to where to even start to not only create a freemium site, but how to transition from client side to server side as far as a technology stack.