I'm going to send you some cards: instruments, a vehicle and two people.
Note your statistics before updating your game (so that your new cards are taken into account) then note your new stats.
Finally, observe the changes that this has induced on: your Starbits earnings during missions and your Skill earnings during lessons. This will already be a way of understanding this game.
The first advice I was given when I started was to rush to level 15. To do this, do lessons, lessons and more lessons. ;).