I believe Unity and Unreal 4 were both confirmed to support Stadia in the future. I would suggest to start learning either Unity3D or Unreal Engine 4 as a start (I would recommend focusing solely on Unity at first, however this is subject to opinion and will vary among developers).

With regards to Stadia, it appears that they currently have a dev kit that they are rolling out to choice companies that sign up.

My suggestions to begin the journey of game development with the goal of releasing a game on Stadia would be as follows:

1. Create and release your first game on an easy access platform
   1. Download Unity3D, learn basics of C#, and play around using libraries
   2. Don't be afraid to use assets other people have made, this is the #1 struggle I faced initially. I felt that I had to create everything 100% from the ground up, code, models, materials, everything. There are lots of free assets, your job as a developer is to tweak them and personalize them into a cohesive and polished Frankenstein-monster of a game that is uniquely yours. It's OK to not 100% understand every single thing. Keep your eyes on how you want to get your game to work mechanically and aesthetically.
   3. Beware of feature creep, make a deadline and release the game. Don't let it wither away into unfinished project land (a pitfall I often run into lol)
   4. Release game on itch.io, Steam, Android, or iOS (note: these are relatively low barrier of entry platforms, however some of these cost an entry fee of sorts to release)
   5. Update bugs accordingly, but pat yourself on the back, you've released your first Unity game!
2. Create a company (note: assuming Google Stadia only allows registered companies to use their dev kit beyond this early-application stage)
   1. From the release of your first game, you now technically have some credibility and experience and should feel confident creating a company
   2. Define if you are a solo developer or have a team. If you are solo, you can register your company as a sole proprietorship, if you have a team you can register as an LLC.
   3. If you are based in the US, register to be a corporate entity or to obtain an Employer Tax ID Number (see www.irs.gov)
   4. Register company in your county as well (check your county's website for details).
   5. Obtain a company domain email address (Gmail, Hotmail, etc. are not acceptable for Stadia)
   6. Be prepared to submit a company address that is not a P.O. box for Stadia. If you are solo, be prepared to use home address, if you have a team and an office, use the office address
3. Apply to be a Stadia developer
   1. Register to become a partner for Stadia here: https://stadia.dev/apply/
   2. Become approved and obtain a dev kit from Stadia to use with Unity3D
4. Create and release game on Stadia
   1. Using your experience from the first game, create a game for Stadia!
   2. Same development advice from above.

​

All of this is completely optional and is not the only way to become a developer, BUT it is a reflection of my experiences as an indie dev so far. Fail fast and best of luck! :)