I started with no modding experience and only very minimal coding experience. Being a total noob, I found this (somewhat outdated) guide a very useful starting point.
https://supcom.fandom.com/wiki/Unit_Creation.
Next was having a list of all the Supreme Commander LUA functions. Since the list doesn't really explain how these functions work, I would then search the FA GitHub to see how others had implemented them (note: you need to create a free GitHub account to be able to search for code).
List of SC Lua functions:
https://supcom.fandom.com/wiki/LUADOC_1.5.3599
FA GitHub:
https://github.com/FAForever
And another useful step that is really helpful is to download a mod that is similar to what you are wanting to do and study their code to see how it is set up. Like, if you want to add your own unit, download a mod that adds a single new unit and see how the folders are organized, etc.
Then, when you try running your mod and nothing happens, pull up the FAF log of the game and search for "error". If there are errors in your mod, it will usually tell you the exact line and function to check.
One more useful resource is the Unit Database, if you are working on units. You can quickly check the GitHub of the unit, which will bring you to the unit's blueprint file. From there, you can quickly access the unit's script. If you aren't sure how to get something working, just check how a current unit that has those abilities works and use that as a template.
https://faforever.github.io/spooky-db/#/
This is a useful guide explaining what does what in the blueprint file.
https://wiki.faforever.com/en/Development/Modding/Blueprints