If you are interested in creating a mod I suggest starting by creating a single new weapon item. (
If you need graphics for the item, check these threads)
Then move onto adding a new X-Com armor for your soldiers before creating an alien unit.
This should help you to get a feel for the way a mod's .rul files are coded.
A good mod to look at to understand the structure of the code is the
Final Mod Pack. Everything is setup in a very easy to read format internally so it is not just fun to play but also makes a good tutorial for modding.
The
Ruleset Reference on the UFOpaedia.org site is a guide that will allow you to understand the code's variables and all the settings that you can make use of while modding.
Do not try to use Microsoft Window's default notepad program to edit your .rul files as it can save hidden characters that will cause you problems when trying to run your mod.
Instead use an editor that understands YAML. (For example Xed is what I use)
There are two extensions for Visual Studio Code that will allow that editor to check your rulesets for syntax errors which can be found
HERE and
HERE.