The first step of modding is to familiarize yourself with the code that make up mods, XML and Python.
XML - This will be the primary technology you'll need to understand to successfully create any form of custom content for the game. XML is used for most Tuning files, which explain how to the game specific object or interaction should function.
Python - A scripting language that powers the simulation code. You will need to understand the rules and syntax to successfully create a Script Mod.
For an in-depth explanation about the different kinds of mods and how they work in-game, I would HIGHLY recommend reading this blog post from Carl Sims.
What is XML | XML Beginner Tutorial | Learn XML with Demo in 10 min
June Hanabi - The Sims 4 Modern Python Modding: Part 1 — Setup
June Hanabi - The Sims 4 Modern Python Modding: Part 2 — Hello World
June Hanabi - The Sims 4 Modern Python Modding: Part 3 — Replacing Code
June Hanabi - The Sims 4 Modern Python Modding: Part 4 — Replacing Class Code
June Hanabi - The Sims 4 Modern Python Modding: Part 5— Extending Lists
June Hanabi -The Sims 4 Modern Python Modding: Part 6— Adding Interactions