Most beginner mistakes aren't about strategy — they're about a handful of automatic rules the game applies without asking permission. Once you know what those rules are, most "gotcha" moments stop happening.
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Attacking with a newly cast creature. Summoning sickness prevents it, every time, no exceptions without haste.
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Casting a sorcery-speed spell outside your main phase. It's only legal during your main phase with an empty stack.
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Forgetting a lethal creature should already be dead. The game checks this constantly and automatically — you don't get to "forget" it.
Almost every beginner misplay traces back to one of a small handful of automatic rules — learn those, and most mistakes disappear on their own.
— the pattern behind nearly every rules mix-up
The rule behind most of these is state-based actions, defined in rule 704.
Comprehensive Rules · Rule 704.1
"State-based actions are game actions that happen automatically whenever certain conditions are met. State-based actions don't use the stack."
Comprehensive Rules · Rule 704.3
"Whenever a player would get priority, the game checks for any of the listed conditions for state-based actions, then performs all applicable state-based actions simultaneously as a single event."
This is why a creature with lethal damage marked on it dies immediately, without anyone casting anything — it's not a triggered ability waiting on the stack, it's a state-based action that happens automatically before any player would receive priority again.
Announce your triggers out loud. Saying "upkeep trigger" or "end step trigger" as it happens is the single best habit for never missing one.
Double check timing before casting a sorcery-speed spell. Ask: is it my main phase, and is the stack empty? If either answer is no, it has to wait.
Trust state-based actions to handle the obvious stuff. You don't need to announce "my creature dies" — if it has lethal damage, it's already gone the next time anyone would get priority.
When in doubt, ask before you act. Confirming a play is legal before committing to it is always better than taking a play back after the fact.