For tools, that’s actually relatively easy. If you’re looking to do gameplay mods you’ll need to get SLADE: http://slade.mancubus.net/If you’re looking to do maps, you’ll need Doombuilder 2: http://www.doombuilder.com/index.php?p=downloads
If you’re looking to do ZDoom/Zandronum/GZDoom-specific maps, then GZDoombuilder is a fork off of Doombuilder 2 that adds a whole ton more of convenient stuff: http://forum.zdoom.org/viewtopic.php?t=32392
For a few tutorials on how Decorate/ACS works, the ZDoom wiki is an incredible resource: http://zdoom.org/wiki/Tutorials
For specifically fiddling with weapons and gameplay functions, WildWeasel has a nice set of written walkthroughs with pictures and commentary: http://gunlabs.blogspot.com/2011/01/tutorials.html
For mapping, Doombuilder 2 has a nice set of already-hosted tutorials showing you how to wiggle through the program: http://www.doombuilder.com/index.php?p=tutorials
Alternatively, if you want something a bit more in-depth, ChubZDoomer’s set up another list of Doombuilder 2 tutorials: https://www.youtube.com/playlist?list=PLCE835098C82D8F24
Enjoy, mate. And good luck.
I wanted to get a look into this too, thanks for your help! Might give it a go during the November/December holidays!