I have tried several shopping cart programs, including magento, prestashop, zencart, opencart, TomatoCart.
I must admit magento is obvious the most powerful one. But I will not recommend it to green hand and if you have no coding knowledge, I think you'd be better to forget about it.
Zen Cart is too old and I don't like the back office design, out of order and messy.I don't like it.
TomatoCart is new but very promising. The admin panel is the most user friendly to beginners of ecommerce, similar to a desktop, clean and nice. The feature is comprehensive as well. I would like to recommend it to all small or medium shop owners. But the team is busy developing core feature, the forum is lacking support at present.
OpenCart is also promising, which is awarded with TomatoCart in 2010 Open Source Awards, but the feature is limited.
Prestashop, nice design with enough features. But the extensions are too expensive. If I choose an open source shopping cart, I will not pay a lot to get a payment gateways or a features which is available in other shopping carts.