Hi everyone, I decided to give it a try, but I have never done it before, and I didn't release any games at all. What are the limitations to assets I should use? And what about the models etc? Should I just stick to Unity built-in assets and make my sprites/models from scratch?

depends, this is my first time too, but I have great knowledge of pixel art and I have a couple of friends for level and sound design, but look at the rules and then it's your decision.

Same

I'm pretty confident tough

Good luck fellow beginner!