It seems, in our recent "forum makeover", that we've moved the helpful stickied thread on useful tools for map creators.
Here is is.The primary tool for making maps is the Voxlap Editor, also known as "Voxed" which is in your AoS program folder. There's another program you can google for, which makes voxed a
lot less frustrating to use called
"Voxed Companion". Slab6 is primarily an object creator, though it can be used to make pieces of maps that you can load into voxed and orient and place as you see fit. I'd start by saying you should print out the voxed help file and become familiar with the keyboard commands.
That said, a great place to start is
"bildramer's heightmap utility" which is a handy-little command line tool that creates terrain which you can then load into voxed to edit.
Also look for FaultCheck's recently-released Blockman2D application, which is a good place to start. Also there's the
"Pyspades map editor" which is a very simple editor which is very easy and intuitive to use.