The basic keyboard command to minimize a window is Command-M; the same effect can be done by double-clicking in the top frame of the window.
If you add Option to the command, i.e. pressing Command-Option-M, then all open windows for the active (frontmost) program will be minimized, each appearing as a separate icon in the Dock. Holding down Option while double-clicking in the top frame of any one of a program's windows will do the same.
Note that this affects only the open windows for a single program; it does not affect open windows for another program.
Multiple minimized windows for a single program can be de-minimized simultaneously by holding down the Option key and clicking any one of the set in the Dock.
As far as I know there is no global minimize command that will affect all open windows for all programs, all at the same time.
Is it possible that the person at the Apple Store was showing that a bunch of open windows for folders, i.e. Finder windows, could all be minimized at once? Since Finder is considered a program for things like this, then Command-Option-M will minimize all open folder windows (provided Finder is the active app) simultaneously.
Hiding windows is a different thing.
When a program's windows are hidden, they seem to disappear but are not minimized to the Dock. The basic command for this is Command-H. Pressing that combo will hide the windows for the current active app and switch active status to the next open app.
The Option key and clicking can be used to do this. For example, with one or more windows open in TextEdit, Option-click on the Desktop - all of TextEdit's windows will disappear (get hidden) and the active program will switch to Finder.
HIdden windows can be made to re-appear by clicking the icon for the program in the Dock, thus switching the 'active' status to that program.
You can hide all windows except for a chosen active program in one action - with that program frontmost (active), press Command-Option-H. You can accomplish the same effect by holding down Command-Option and clicking in a window for the app whose windows you want to remain visible - windows for all other open programs will be hidden.
Also, I would like a shortcut to get to the desktop. This is part of what I also am trying to do.
If by 'get to the desktop' you mean the normally visible desktop, just click anywhere outside any open window - this will make the Desktop (i.e., Finder) active.
If you mean to open the folder named Desktop, meaning the folder by that name which resides in the user's home folder on the HD, you can change the action for the Finder icon in the Dock to have that happen. To do that, make Finder the active app, then open its Preferences (in the Finder menu when Finder is active). In that screen, click the General item if it is not already selected. In the middle of the General page is an item "New Finder windows open:" with a pulldown menu just under it - select "Desktop" from that menu; close the prefs window. Now test - click the Finder icon in the Dock - the Desktop folder should open.