Nunit or unit testing for asp.net website

NET website. I need to do unit testing for my code. Aspx.cs has few methods. but all logic is in app_code, Business logic class. Also, there's a lot code behind in the website. it has validations, creates html page, sends email.

Please help me in doing the unit testing for my website. I tried Nunit but we need to do it in web projects. how do i do for a website?