Why do you want to start with a web-application? I think thats too hard at the beginning.
Maybe you have a specific problem you want to solve.
Thats really good, because nothing motivates better than buildng something useful.
But, if you really have zero things done before, better start with some tutorials. A great way to get started is
Solve the problems, have a look at how others solved them, you can even get help by a mentor.
After that you could implement parts of your application, but without the web-part. This could later become the contexts (How would you explain Phoenix Contexts to a newbie?) of your web-app.
