Conquest the first night is usually the most popular choice. It’s has pretty good free cards and packs every year. Its three inning games vs the CPU. Probably watch a quick video first but it’s ez.
After a few games go look at “Team affinity” and see all the progress your making with the cards you got, then choose which teams cards you really want to unlock and get some players from those teams (all free btw)
Then go to the XP path and see how far you are and how many amazing cards are in it (again free)
Then go back and finish conquest.
Do what you want with your $, you can only buy stubs, stubs buy packs or cards, the cards are the smart option, the packs are the other option.
If you do get a Diamond, see what collecting the whole team rewards you in “collections” then look in collections for more fun.
Then play the game until you can buy all the cards, complete all the collections, and then come back next year.