Here are the steps as I understand it:
- Produce a nontrivial, working game for PC.
- Provide a way to receive revenue.
- Fill out the Steam Direct form and pay the 100 USD fee.
Some of those steps can be broken down further with practical business considerations:
1a. Make sure you can complete the game. You'll need enough money to pay your programmers, artists, composers, and quality control for any game larger in scope than the
low-effort "fake games" that have become common on Steam.
1b. Ensure your game works on a wide variety of PCs, with both integrated and discrete GPUs, and all supported versions of Windows (currently 7, 8.1, and 10), as well as at least one of GNU/Linux or macOS. Porting to more than one PC operating system and having a low-detail option help keep your game flexible enough to reach a wide range of devices.
2a. Consider forming an LLC or S corporation. This costs a bit but helps to manage risk in certain scenarios involving errors and omissions. If you go this route, you'll need a bank account and tax ID for your company.
If you're just starting out, you might be able to make up these costs by first selling smaller games on Itch before Steam, as
rainwarrior recommended, as Itch has smaller setup fees.
EDIT: Pyramid style (summary up front, then details); remove suggestion to try to publish on PlayStation 4 in parallel