Visual Basic Studio

I am creating a shopping cart to purchase books. The book is selected from the choos a book drop down and added to the shopping cart list with the press of a button. The books are then totalled to the right, and a credit card is entered in to pay for the book. Where would i start in coding this function? The credit card would need to be a valid number or an error message will pop up.

x Final Project vb Form 1.vb Design x Main Form.vb [Design] Book List vb a Order Form File Choose a Book Add To Cart Shopping Cart Order Totals Subtotal: Label 10 Cart List Label 11 Tax Shipping: Label 12 Total: Label 13 Credit Card Info CC Type CC Number Exp Month Exp Year Reset Check out Eg MenuStrip1 Book List 2000Dataset BookListBindingSource Book ListTableAdapter Error List