With the Map Editor players can create their own maps and scenarios. This document is supposed to describe how to use the Editor with all of its functions (actually it misses a lot - please help to improve it). You can also view an example how to create a map in the EditorHelpTutorial. A very basic example for a scenario map can be found here.
When you start the Editor you see a plain map with nothing on it yet and nine buttons at the bottom edge of the screen.
Clicking on these buttons (or using hotkeys) has following consequences respectively:
pressing I activates information tool.
For further information on any functions please follow the proper link. Aside from the above, this Help also contains a TriggerAndEventsReference as currently available.