Don’t Fast Upgrade cards unless you’re at the credit max (think it’s 5000). If you only play to do your daily missions, you’ll have more Credits than what you can use, at least for a good while.
Getting cards is just playing and upgrading cards. You’ll end up upgrading all the cards, the very first upgrade is the best value, after that, they’re all equivalent.
Don’t be afraid to retreat if you think you’re gonna lose.
For the most part you’ll be playing against people who have a similar card pool to you. Or bots.
Game has well defined points where the meta changes dramatically, for example a popular starting deck is a bunch of 1 cost cards, Kazar and Blue Marvel, that works really well until you reach a point where people unlock Killmonger, that kills all 1 cost cards.
Have fun, game is as much about playing the locations as it is playing your deck.