If you will be selling products online and collecting payments via credit card, you will need a gateway and merchant account. The most trusted and reliable gateway is authorize.net. They offer a secure gateway that communicates with your site via ssl and the credit card companies, verifying, authorizing and transferring funds in a secure manner. We have become a preferred reseller.
If you are looking for a low-cost option, consider paypal.com. They utilize their own gateway, merchant account and even shopping cart. It requires extra steps from your customers, but is a functional and low cost approach.