First you design the application data model - the authentication probably maps to some kind of role system - a
wrapper facade[
^] around the datamodel, hopefully serverside, ensures that the user do no get unauthorized access. This layering of functionality may seem foreign at first, but it's an excellent example of SoC.
Then you design the application user interface - don't think about data at this point, focus on the workflow, then aesthetics.
When you have a decent UI, design your View-Model(s) in a manner that supports that UI. Try to avoid using prism and similar libraries, later on when you are familiar with wpf - you will be able to make informed decissions about when it's appropriate to use such framworks.
At that point, it's a good idea to take a look at
caliburn[
^]
Best regards
Espen Harlinn