The stylesheet should be in includes/templates/YOUR_TEMPLATE/css/stylesheet.css (where YOUR_TEMPLATE is the name of your custom template).

If you have (or get) the Firefox browser, get the webdeveloper addon which will allow you to tinker with the stylesheet and see the changes on the fly. You'll have to save the stylesheet locally and FTP it to your website.

When using Zen Cart, you need a good text editor like Crimson Editor or Notepad++ and an FTP client like Filezilla. All are free downloads on the net.