I would start by using some tool that would test the application through the UI. There are a number of tools that can be used to create test scripts that simulate the user clicking through the application.
I would also suggest that you start adding unit tests as you add pieces of new functionality. It is time consuming to create complete coverage once the appliction is developed, but if you do it incrementally then you distribute the effort.
We test database interactions by having a separate database that is used just for unit tests. In that way we have a static and controllable dataset so that requests and responses can be guaranteed. We then create c# code to simulate various scenarios. We use nUnit for this.