There are many different ways to setup an ecart? First you should ask yourself how do you want the payments/sales to go? Just with Paypal, very common and easy to setup. If you do not have Paypal yet take a look at paypal.com
To put it simple PP will provide you with html codes which you paste into your website, job done.
Or have a look at Dreamhost partner CafeCommerce (link in the panel ECommerce).
After you are more detailed on what you need/want, we can be more clear here.