Stuff you need

• A Supreme Commander map editor. There exist two versions:

• The official Supreme Commander map editor, previously available from The Vault inside your GPGnet client. Now, with GPG down, it can be found with other documents here.
• HazardX's scenario editor, available on the SupCom forums

• Some sort of texteditor, to edit the script files. Notepad is ok, but you'll be better off with Notepad++ as it features syntax highlighting.
• A lot of time >.<

Stuff that comes in handy

• an image manipulation program, to edit/mirror your heightmaps and texture layers, e.g. Adobe Photoshop, GIMP, Corel Paint Shop Pro, or paint.net.
• a few other tools and pieces of information that are available on the FAF forums, for example those here.

Advanced Map Editing Tools

Thygrr's Box of Cool Tools

Thygrr has made a cool posting in the forum about some tools he uses: Thygrrr's box of cool tools

Resource Management Tool

This program views and modifies the lua files of SupCom maps, showing the user where the markers of the map are in relation to each other.