need help with online shopping cart with mvc

Hi,

I have an application and I am new to programming where we have diff products I need to develope

an checkout for online application

The initial product catalogue will contain the following items

prod Name Price

pd1 testprod1 $500.00

pd2 testProd2 $1099.99

pd3 testprod3 $149.50

pd4 testprod4 $30.00

there are some offers on the products

There will be a 3 for 2 deal on pd3 - for example, if you purchase 3
pd3, you will only pay for 2
The pd1 will have a bulk discount applied where the price drops to
$449.99 each if someone buys more than 4
A pd4 is bundled free of charge with every pd2 sold

The interface to the Checkout looks like this (shown in C#):

var co = new Checkout(priceRules);

co.ScanProduct(item1);

co.ScanProduct(item2);

co.Total();

how to get started on this application using TDD.Any help is highly appreciated.