Identify High-Level Functions First
High-level functions are those functions that are most 
important to the central purpose(s) of the site or 
application. A test plan would typically provide a 
breakdown of an application's functional groups as defined 
by the developers; for example, the functional groups of a 
commerce web site might be defined as shopping cart 
application, address book, registration/user information, 
order submission, search, and online customer service chat. 
If this site's purpose is to sell goods online, then you 
have a quick-and-dirty prioritization of:

shopping cart 
registration/user information 
order submission 
address book 
search 
online customer service chat