Within your Google Analytics account you can go to Admin > Goals, and setup a cart abandonment funnel to track what part of your checkout process is determing customers from completing their order.
1. In the first section for Goal Setup select Custom.
2. For the Goal Description enter whatever name you want, such as "Completed Their Order", then select Destination for the type.
3. In the Goal Details you will want to enter the base URL for all steps of your checkout process. The URLs to enter should be everything after your domain root. For example, if your domain is http://mystore.com, and when you go to your shopping cart, the URL is something like http://mystore.com/cgi-bin/sc/order.cgi, then your Goal Details would be setup similar to the following:
Once you have added your Goal Details, save changes, then check back in a day or so to see that your cart abandonment funnel is tracking correctly. You can see the cart abandonment goal overview under Conversions > Goals.