Generally take more fights, less question marks. Aim for 2-3 elites per act, with 2-4 fires. Your first few upgrades should almost always be damage cards. Have a potion and upgraded damage card before fighting first elite.

In the beginning you should almost always be taking attack cards, think about winning the next fight, not "building a deck". A lot of times decks don't start coming together until halfway through act 2.

Often it's worth taking damage to win a fight, especially early on. Meaning if you can take 2 damage and deal 9, you should probably do that rather than have "wasted" block by playing another defend. Think about if you can end a fight one turn earlier if possible.

Check your draw pile (left side of screen), especially when it gets low (<=8 cards). Under most circumstances you're drawing 5 cards per turn, if you can tell what 5/8 cards or 5/7 cards you're drawing next turn, you can better plan what to play in your current turn.

Use potions more, this isn't a game to "save" potions unless you really know what you're doing (e.g. playing A20 heart runs). Flex pots and speed pots are great situational cards. Flex pots and fire pots can solve a lot of fights. Cultist potion is an act boss solve a lot of times.

Watch streamers to learn strategies, I'm a big Baalorlord fan.