The first step is to install Zen Cart, preferably on your local computer (instructions in the wiki, under Customisation.)
Install the Demo products and play with the whole thing. This will give you the best idea of what it can do; only then can you begin modifying the look and feel. Read FAQs, forum and wiki, and if you're serious about this your job will be easier if you buy the newly published manual (link at the top of the page). I have it and it's very good, especially for a beginner.