The demo products and categories (in that order) can be deleted in admin, though I would advise keeping them until you have studied them all thoroughly - they all have some teaching purpose to show you how various Zen Cart features work. You can disable the demo categories in admin to get the navigation menus clear for the time being.
A good way to start transforming the look & feel of your store is to install one of the free templates from Plugins (look at (sorry, site offline) to see them live); you can modify its colors, fonts, sizes, etc. from there by editing the stylesheet. Many basic layout features can be adjusted in admin. Look at the tutorials (FAQ button at the top of this page) to learn about setup and beginning customization.
You don't need to know any HTML to do basic customization. A bit of knowledge about CSS will make it easier to start tweaking things. See online tutorials like https://www.w3schools.com for basic CSS and HTML tutorials.