What to unit test?

[eluser]chobo[/eluser]
I've never unit tested in an MVC framework so I'm not sure which area(s) (model, controller, view) should be tested. Could someone explain how they test their projects? I'm using simpletest, but I'm not sure how to test the controller with it. Also, where do you guys store you test files and how do you run them (from the controller folder?). Thanks in advance.