The 3 most important things to know when customizing Zen Cart are - html, css, php.

For the most part you don't really need to know much about php unless you are trying to create some advanced functionalities. Most of the time you will cut/paste php scripts and move them around if necessary. USE THE FORUMS!! Just about every question has been asked and answered on the forums. Many people are new to a lot of this stuff and manage to get very nice web sites published.

Checkout http://www.w3schools.com/ for some great beginner tutorials and information.

Zen Cart runs better on Apache, PHP, MySQL. You should consider installing Apache (for Windows) instead of IIS. Depending on your current site, you'll probably find it better to use Zen Cart for your entire site rather than just for shopping cart functionality.