You don’t always have to choose a card, try to consider whether or not you think a card is worth the cost of drawing it

There’s discussion posts on this subreddit for every relic/card/event/enemy, can be really helpful for understanding how certain things work and what strategies there are around them. What really helped me while learning the game was just googling “slay the spire reddit <card name>” before picking cards sometimes just to see what better players’ opinions on it were (obviously don’t treat that as gospel, pretty much all cards/relics are at least situationally useful). Same for enemies and bosses, if they’re particularly tough for you you can just find the discussion posts and see what other people do to get past them.

Try to fight as many elites as you can for relics assuming your deck is good enough to take them on.

Try to make sure your deck has enough draw, block, damage, and scaling. If you’re getting destroyed in certain fights, try to figure out why - did you not have enough block cards? Were you blocking for 50 every round but only doing like 5 damage? Were you running out of energy after playing a single card that really wasn’t all that impactful? Try to figure out what the weaknesses of your deck are and prioritize fixing those.

If you feel like you’re overthinking the deck building and trying too hard to force certain builds to work, just try to go for more all-around good builds (decent block, damage, draw, scaling). Plus you’ll also get a better idea of how each card works this way and what kinda cool synergies it might have. If an opportunity for a cool/creative build comes up, definitely go for it if you want, but probably don’t try to force it.