In Potlatch2, what symbol should I use for a drug-store like "Schlecker"

You are right. Chemist would fit it perfectly. But there is no icon to drag it onto the map. And the icon pharmacy is not the right icon.

Please use the comment function instead of adding an answer to your question :).

What editor are you using? Potlatch? JOSM? Some other? The Icon in the editor doesn't matter as it is independent from the rendering process. The renderer have their own icon (and sometimes even don't have one when the editor does). Just adding a node or a building outline with shop=chemist (and maybe additional tags if known) is enough.

Ok, now a comment. :-)

I am using Potlatch. BTW: I am using it since one day. Thus, this stupid questions. But how to add a single node? And where to write "shop=chemist"?

For learning how to edit with Potlatch2, please take a look at the wiki.

You can add any tag you like in OSM. To do this in Potlatch2, select a node or create a new one and click on "advanced". There you can add the key (here: shop) in the left column and the value (here: chemist) in the right column.