You can run the world editor out of the WC directory (macs dont have a start menu thing to dthey? I've never used one) using the world editor itself is pretty straight forward, you have 3 basic things:
Terrain
Objects
Triggers
Terrain is well, terrain, it's what people can see, you can also add "Doodads" which are un-controllable objects that can add to your terrain. You should try experimenting with terrain as a first step into mapmaking (you use the terrain palette to do this, just press "T" at any time once inside the editor).
Objects are things such as Units, Doodads, Abilities, anything that exists is basically an object, mostly people refer to objects as Units and Doodads though (other things such as items and abilities are much more complicated). This is the next step after terrain, or the first step depending how you want to learn to make maps (I recommend learning to make decent terrain first, maps with shit terrain are never fun).
Triggers are anywhere from simple to advanced depending how you use them, for example if you look at that tug of war map the triggers are relatively simple and straightforward (short too to help make them easy to read), but in other maps, take my haunted house map, some of the triggers are enormous and hard to understand (the spawn trigger spans four of my computer screens, and I used a 1600x1200 display). This is the hardest thing to learn and master in mapmaking, but given time it becomes easier.
If you'd like I can actually write a tutorial for how to make a map from scratch, maybe make a simple CTF map step by step or something...