As a game designer who hires teams to develop mobile games, this is what I look for.
- Expertise in an applicable dev environment. My teams use Unity, but find what's prevalent for the type of games you're passionate about. And make it your life's mission to master every part of it.
- Competence in game design. Yes it's a separate skill set, the psychology of designing fun and effective play spaces. A good dev will implement my crazy ideas exactly as written. A great one will push back and offer a better idea that improves the game. My favorite book on this is Brian Upton's "The Aesthetic of Play."
- Great team fit. I value solid listening, speaking, and writing, a positive attitude, and a willingness to speak up and back your assertions with evidence. There aren't specific courses in most of this, but you can ask good friends what your strengths and weaknesses are, and find mentors who can critique and help you improve.
This professional recommends the following next steps:
- Complete tutorials in a dev environment
- Complete a game even if it's a simple clone of something you like to play
- Watch people play your game, so the next one will be better