You can learn following to accomplish a decent backend with mongodb :

• Learn mongodb with some latest aggregation features to make sure the your queries are optimized. mongodb has great documentation which can help you. Install mongo shell and play around to get hands on it.
• Make sure your have good cluster-replica management to make sure your data is backed up properly.
• Extremely important, make sure you add indexes on the fields which you find are reference points for majority of your queries. Trust me, adding a simple index to a commonly referenced field makes a huge difference. Also, just as an addon, check what type of index you would need based on your requirement. There are various available in mongodb documentation.
• Make sure you structure your collections in such a way that there are not many $looups(inner joins) however still making sure how you can keep it scalable. It totally depends on your application schema though.
• Try hands on Mongodb Map-Reduce as its very handy.
• Install RoboMongo, a GUI client for mongodb if you are used to stuff like phpmyadmin.

Node js(optional if required) : 1. Learn express which is really easy. It will help you to get started with basic routing, cors handling , middlewares and such http-web related stuff. 2. Mongoose, which is a mongodb package for node. There are many others, however if you are familiar with ORM then mongoose is really handy to get you going with simple steps. 3. Authentication for RESTful APIs : There are many options available to be honest. However, based on my experience, you can use passport-jwt, which is really simple and effective.