As mentioned, PayPal is one of many ways to go. Other companies offer just as good or better rates and you won't have to send your customers away from your website to process their credit card payment. You can also check with the bank you already do business with to see if they offer a payment gateway.
Your payment gateway needs to work with the shopping cart that runs your website. Pay-Gate supplies a cart (Easy Cart) free, and works with 99% of the standard shopping carts available on the Internet, so there’s a very, very good chance it will work with your site.