Minecraft
The most-played PC game in history and the best demonstration of what PC gaming's mod ecosystem can do. The base game is approachable and complete; the Java modding community has produced thousands of additions. Learning to install mods, adjust settings, and customize your instance is a low-stakes way to learn the broader PC gaming skill set. It runs on virtually any hardware made in the last decade.
Fortnite
Free, cross-platform, and designed for accessibility. Zero Build mode creates an entry point with no mechanical gatekeeping. Epic's recommended settings documentation is some of the clearest in the industry for new players trying to maximize frame rates on a budget machine.
Cities: Skylines
One of the best introductions to PC gaming's long-form management genre. Learning to install and manage mods through the Steam Workshop is the easiest version of that process — one click, automatically downloaded. It is a good gateway to understanding how PC game libraries and mod ecosystems work.