MVC 5 Wanting to testing with SpecFlow

I want to build a mini Asp.net MVC 5 application which only has a login page. I need to use BDD so I decided to use Specflow but how can I write my given,when,then test methods that will check when I enter username and password is valid or not.

In short, I want to use BDD specflow testing when I enter login information(username,password).I don't know what to write in controller and how to take data from view to give that in specflow methods, can you help me? Do I need to use Assert for example in methods, or given,when,then methods will be in controller ?