welcome to OFPEC - you've come to the right place.
you'll probably already know that ArmA is built on the previous success of operation flashpoint (OFP). the two are quite similar editing-wise, but with enough differences to make things tricky. since ArmA is relatively new you'll find only a few handfuls of resources specifically for that game. however, the principles of mission editing such as how to place units, how to get them to do stuff - all that is still the same.
my suggestion therefore is that you start with the FAQ. even if you only skim through it you'll begin to see the mechanics of both in-game editing and scripting, which looks a lot more daunting than it really is.
then move onto the COMREF. again, skimming it will give a good idea of the commands available to editors.
finally, get into the guts of it with the tutorials section of the editors depot. they are all gathered from the OFP era but a great many of them will still be relevant.
you'll find we have members representing a wide range of editing abilities, and we always welcome those who are just starting the journey - we were all there once, so don't worry about it. a little light reading through the resources mentioned above will give you a solid start.
once again, welcome to the site.