Hi and welcome to OFPEC!
The best way to learn is to do. That is to say, simply try making a mission. Very soon, you'll bump into something you don't know how to do ("How to check if everyone in the group is dead?") - then you search around for an answer ("{alive _x} count units (group unit1) == 0"), and failing that, you ask the question here (with an informative thread title and a description of what you need done and why) and hopefully someone can help you. Then you continue on, until you bump into another problem. Rince and repeat!
Aside from what SBG there told you, there's another thing you need to keep in mind. The most important tool you, the mission editor, has, is EXPERIMENTATION. All the commands in the game are already listed, right here in our very own
COMREF or in the equivalent on the BIKI:
Scripting Commands. You can do almost anything with clever combinations of those commands - that's all scripting is, really. Just combining commands in a clever way. Usually the descriptions can give you a hint of what they do and how they work, but the best way is to
try them yourself! Not sure how "alive" works? Add a trigger that has the condition: not(alive unit1), and the on activation: hint "The unit is dead!". Then go ahead and shoot poor unit1 and notice your hint popping up.
So yes, go forth and edit.
And have fun!
Wolfrug out.