Your question is very generic, it's like asking how to write a novel, or how to make a computer game.
Look in the faq at the tutorials links. Also look at the documentation that came with the game (access it from the help menu in the editor).
A very handy thing to do is to open up one of blizz's maps in the editor and look at that. The maps need to be extracted from war3.mpq. The program you need to open up war3.mpq is found in the faq. The maps aren't hard to find in there, just look for w3m files and you'll find them easy enough.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory
I will be AFK for a couple of days you guys hold down the fort while I am gone. I will be checking in on my phone but that will severly limit me. Be back saturday!