Newbie here, testing the waters to see if I want to help

I thought I’d try to add a furniture shop to a map. Just to see if I can, and to see how editing OpenStreetMap works.

Well, I find on going into an edit screen options under “Drag and drop points of interest”. There are options for cafe, cinema etc. but nothing for a furniture shop or even a general, non-specific shop.

So is it impossible to add a furniture shop?

No, it’s not impossible. The drag-and-drop presets are just some common ones.

To add anything else you can just double-click to add a point, then use the little ‘+’ icon in the bottom right to add the tags you want to it. There are lists of tags on the OSM wiki - search for ‘Map Features’. Or you can press ‘F’ in Potlatch and type ‘furniture’ to find out what other people are using.

shop=furniture is documented on the wiki, and I’ve used that myself a few times