Assuming you already know how to access the Smart Card reader's API, and you can successfully read and decode that data, your next step is Authenticating that data.
Again, you haven't given me any information here, so I'll assume that you have a database, against which you will need to validate the data you got from the smart card.
If the above returns true then it's simply a case of calling FormsAuthentication.RedirectFromLoginPage
, or if you want a more granular control of the Authentication Cookie, then you can create your own FormsAuthenticationTicket