Steam sells 1000s of games, from low-end indie games that are smartphone ports, to the latest "AAA" titles such as the newest CoD WW2 FPS, or Assassin's Creed Origin.
Every game has its own requirements in terms of CPU, RAM, and Graphics Card. Some games require a controller. You can use a Xbox or Playstation controller, or Steam's own controller. Some games also require a VR headset such as the Oculus Rift or HTC Vive. I'd avoid the VR games for now - the equipment is still pretty expensive and the games are still pretty rough.
A decent gaming desktop is going to cost around $800-1200, depending on the actual parts you choose.
In general you'll probably want something like this:
Processor: Intel i5. (The i7 is more powerful but doesn't do much for gaming - save your money.)
RAM: 8GB minimum. 16GB is becoming more common but you can easily upgrade from 8GB to 16GB later on.
Graphics card: nVidia GTX 1050 Ti. The current sweet spot for performance vs. budget.
The graphics card is the most important item in a gaming PC since it's the thing that is going to process all the graphics and effects for the game. This is also why it's often the most expensive component in the PC. The 1050 Ti will handle most games at 1080p at a decent frame rate. However some of the newer games may demand more processing power, meaning you'll either have to lower the resolution or start turning off additional features in order to keep the frame rate up. Still it should serve you well, while not busting the bank.
As for hard drive, this where you will store your operating system (Windows) applications (web browser, word processor, etc.) and games. If you want a bit of extra performance, consider a SSD instead of a normal hard drive. A SSD is basically a larger version of your USB thumbdrive. It's a lot faster than a hard drive, but, also a lot more expensive. Most people that go with a SSD stick with a 128GB or 256GB size since the larger ones get really expensive. Otherwise, a 2TB hard drive is pretty cheap, and will also give you plenty of space for games before you have to start deleting them. Once you buy a game on Steam, it's part of your account. This means you can download and delete the same game over and over again because you've already paid for it.