Sign up as a Steam partner at partner.steamgames.com
You'll need banking details, tax information, etc to complete the digital paperwork and you can find more details at partner.steamgames.com/steamdirect
Once you're set up, you pay a fee to get an appID which let's you set up a store page (requires approval before being made public) and a game build.
The game build needs to be approved before release too, but the testers basically just make sure it runs and matches the store description.
Once you're signed up there is a lot of documentation and a developer forum if you need any help.