Target screen resolution
Another good thing to fix at the start is the game's target screen resolution. Unity does allow you to create a game that supports dynamic resolution changes. However, we will fix the resolution for now. This will make things easier. Yet again, baby steps forward! We are learning in the right order instead of jumping into deep waters.
This is it. Very simple! Almost feels as if it's not worth planning at all? Trust me, it is. The truth is that a game is created in your head first. It's best to take that vision ahead and create at least some documentation from it. Make this a rule: note down your ideas as soon as you have them.