Most web applications don't do much more than just processing and presenting different kinds of data.
I would start with exactly that: What data do I want to process?
After that, you can start modelling how this data will fit into a database best.
Then you should also think about how you access this data-does it give you any hints for optimization in the storage?
I would design the application itself either at the end or even parallel to it. The application design should be independent from the database model. It's only the code itself which will in the end access the database.
But, web applications also tend to grow. So an evolutionary model, where you add new fields or tables to the database and build new code around it, is very common.
