Securing merchant accounts and offering secure credit card processing on a website requires an SSL certificate.

SSL Certificates can be purchased from a number of sources such as verisign and Thawte. These are both popular names in secure credit card processing. The usual cost of merchant SSL certificates is a few hundred dollars. 

How To Use an SSL Certificate

SSL certificates can be installed on a web server without much trouble after obtaining one. Once the SSL certificate is installed the site can then be accessed from https:// ...

This indicates a secure connection. Your credit card processing page will have to be in a https:// part of the server. This will ensure that credit card details are sent through a secure merchant channel. 

There are a number of web hosting companies that offer SSL integration and can configure a shopping cart to run on a secure domain.

3rd Parties

The alternative to creating an order page on the site itself is to use one of the popular 3rd parties like PAYPAL to handle credit card processing. In these cases the customer fills out all their details except for their credit card details.  Upon clicking the next the customer is taken to PAYPAL to complete the purchase and then taken back to the website.

This process is not required a merchant account or an SSL certificate.