Things to know in rails?which help me to be a better rails developer

Aside from practice that will come by building stuff I'm not sure there's some secret sauce about Rails. Go through the official Rails guides and the Rails tutorial and you'll learn most everything there is. There's no need to rush on learning all gems and libraries until you need them but get familiar with what the gems that your app is using do...

The rest is web development in general. A few concepts you might want to get familiar with:

• HTTP(S) and how it works, after all Rails is a framework that responds to requests and issues responses
• relational databases, what are they, how they work, the basics of SQL. Even though you're going to access data through a library (Rails's Active Record) you'll likely need to know how SQL works anyway, because that's what AR ultimately builds
• caching. Mostly two types of caching: server side caching (as in storing data in memory, on a disk, in a DB or in a caching server) and HTTP caching (how to tell the browser not to re-download content each time they ask for a page)

I think this is already a lot of stuff to get started, there's more obviously but I believe that learning these fundamentals and going through Rails docs (which will expose you to other concepts like testing, auth, and so on) is going to be helpful.

Enjoy and try not to get overwhelmed (said the person who just dumped on you a lot of things to learn :D)