How tricks are played
The rules are simple: lead one card, play clockwise, follow suit when possible and determine which card wins.
One trick, step by step
Following suit is compulsory
You may not discard from another suit or play a trump while you still hold a card in the suit led. If you have no card in that suit, you may play any card. Playing a different suit without trumping cannot win that trick.
The suit led controls the trick
Suppose diamonds are led in a No Trump contract. A spade ace played by somebody who has no diamonds does not beat even a low diamond, because only cards in the led suit are eligible to win. The spade ace is simply discarded.
Basic card-play habits
Pause before playing
At trick one, declarer should count likely winners and losers before calling a card from dummy.
Watch every card
Notice which suits players cannot follow. A shown void can explain where remaining cards are located.
Keep entries
A winning card can be valuable because it lets you reach the other hand later, not only because it wins now.
Think as a partnership
Defenders should consider why partner selected a particular suit or card.
The essential rule: follow the suit led whenever you can. Everything else in card play builds on that foundation.