I'm getting started out creating a website where users can store and get (on user request) private information they store on the server. Since the information is private, I would also like to provide 256 bit encryption. So, how should I go about it? Should I code the back end server stuff in node.js or Python, since I'm comfortable with both languages? How do I go about providing a secure server to the user? And if in the future, I would like to expand my service to mobile apps for Android and iOS, what would be the process?
Please try explaining in detail since that would be a great help :)
You don't need to create your own encrypted communication protocol. Just serve all traffic over https.
If you also wish to encrypt the data before storing it on a database you can encrypt it on arrival to the server.
Check out Express.js for the server, Passport.js for authentication and search for 256-bit encryption on npm. There are quite a few implementations.
Build a Server to Receive and Send User's Private Information