Use .NET to authorize credit card?

[Lecture Snoddddgrass]

Hi,

I'm designing an application in which the user will enter a credit card
number and expiration date. I want my application [which will be a .NET
service running on my server] to check with the credit card company to get
authorization for billing. If the authorization goes through, my app will
supply the user with a password.... A human being will be responsible for
actually *billing* the person. All my app needs to do is check for
authorized funds. How do I even get started with something like this? Do I
need to call VISA? Is there some special API that is used for this type of
thing? I'm envisioning a simple method that looks like this:

bool IsCreditCardValid(String creditCardNumber, DateTime expDate, double
amount){
//What do I do here?? :(
}

David