If you already have an e-commerce solution setup (such as 1ShoppingCart, PayPal, Google Checkout or any other system), you simply want to create a new product in your cart and follow the specifications to setup the product.

If you don't yet have an e-commerce solution, the easiest thing to do is to setup a PayPal account and use their Merchant Tools. Inside of the merchant tools area, you'll find Buy Now and Subscription buttons which will allow you to setup the single purchase or payment plan purchase options. However, using PayPal means that you can not send them autoresponders. You will need a separate system for that and you'll have to add them manually.

A second option is to use 1ShoppingCart with PayPal (if you don't yet have a merchant account like Authorize.net). Here, you can't use payment options so you will need to go with the one pay option only but you will be able to add them directly to an autoresponder once they've successfully purchased.

A third option is to secure a merchant account and connect that to 1ShoppingCart. This would allow you the same benefits as above but you can also add payment options.

Once you've decided on your system, create the product and add the buy now button code to your sales page in the appropriate place.