Is the goal to build the actual workings of the app (you want to learn CRUD app development)? Or do you have a product/service that you want to sell using the app (e.g. you want people to be able to create an account and buy and sell).
In the first case just search for tutorials or best practices on building apps. For the second case, just use an off the shelf application.
Neither, really. I'm a developer and nothing in front of me is completely new, but taking a product from idea to completion has a fair amount of Coastline Paradox involved. Not looking for a "how do I do this" solution, more along the lines of, if I want to tackle this, here's a list of tasks I'll need to make sure to cover.
For example, creating a user account creation flow is easy enough, right? You just need a form for a username and password. And password complexity checking (client-side for UX, and server-side for security). And something like bcrypt server-side for hashing (and make sure to follow best-practices in hashing and storing). And you'll want to verify emails to cut down on spam, so you'll need some sort of way to send emails, probably a service because rolling your own email server is....a lot. Oh and where are you going to actually store the user data? MySQL/PostgreSQL or something like NoSQL? Managed database, or are you going to handle the setup, backups, hardening, etc yourself?
Congratulations, we've finished creating a user account! Except for password reset, session authentication tokens, rate-limiting login attempts, CAPTCHAs, and everything around the admin side of handling bad actors (account suspension, IP and/or email blocklists, etc etc etc). ;)
In the first case just search for tutorials or best practices on building apps. For the second case, just use an off the shelf application.