Newbie Question

I am just beginning to learn Zen-cart for my company. So here is my situation. The Whole site is not a e-commerce site, and it is already designed. We are going to sell our business lectures. there is only 10 items being sold. I want to implement the shopping cart in a iFrame on the page already designed where we will be selling these lectures. So How do you design a shoppingcart in Zen-cartthat can be stuck in a iFrame?

You don't. Most payment processors won't accept transactions which originate in an iframe, for security reasons. You can make Zencart your whole site (strongly recommended) or put Zencart in a subdirectory like /store and link to it from the main site.

as you already have a layout i'd recommend creating a zencart template to match. Once you get used to the template system its quite easy. Try downloading a free one and play about editing it. If you do decide to use an iframe (not recommended) then you will still need to edit the templates to remove all the default design

+1 on the previous comments.

The most common approaches I have seen are:1. Separate Sites. Leave existing website alone and create second website.
2. Install Zen Cart in a subfolder and update template to match site.
3. Convert entire existing site to Zen Cart using EZ-Pages or creating custom pages.

1. Link to second site for purchases. Personally I do not like the Separate Sites option. However this is a simple method. Another bonus is the site look could be different as most people are used to seeing different site designs on different domains.

2. Probably the most straight forward option. Simple and effective. Just link the existing content to the Zen Cart installation.

3. Probably not worth the work for just 10 items. If most of the existing website content could be moved into category / product pages... With only a handful of other pages (can use EZ-Pages)... It still might be worth the effort. But then you have to deal with other things the above two options do not... such as 301s.

Other thoughts with Option 2 or 3:
These options will require either finding (or buying) an template close to your current website design (and modifying it to fit your needs) or hiring someone with web design skills and a familiarity with Zen Cart's template system to create a template matching your existing website.

Another task you may wish to undertake is making the URLs between your existing website and Zen Cart match (or at least follow similar conventions). This would not be a simple project, but the end result would be human readable URLs which match your existing site and make it appear to be more tightly integrated. This would require doing some URL re-writing. Warning: URL re-writing is not supported by Zen Cart out of box (or the Zen Cart team) and frowned upon by a portion of the Zen Cart community.