Use the help file, it's actually an internet link. It'll explain to you what each part of the trigger does, alot about the different functions possible, a bit about the object editor. WorldEdit isn't as complicated as it seems really, it just takes some time to learn.
I'd also tell you to ask any questions over
here, but Blizzforums is down right now. There are alot of people there that could answer your War3 questions.
Well I'll try to explain the three parts of the trigger.
Events: These are similar to the conditions of Starcraft triggers. You can have any number of them but unlike in Starcraft where they all must be true for the trigger to fire, only one must be true in WorldEdit. As long as one event takes place, the trigger will move on to it's next part, the conditions.
Conditions: After an event takes place the conditions are checked. All of them must be true for the trigger to fire, similar to Starcraft's. I believe that under normal circumstances the conditions will only be checked once when an event takes place, but there is a function that can force the trigger's conditions to be constantly checked once an even happens.
Actions: Same as Starcraft just you have tons more options.
There are a few other parts the trigger editor, like variables, but you're better off understanding how the triggers fire first.