Those who want to know “how to count cards at bridge” probably mean “how to read cards at bridge”, as that would be the appropriate term for the game.
I don’t play bridge much at all. The following information therefore is based on what my bridge-playing friend has told me about so-called “card counting” at bridge.
It’s not a myth; you can indeed count cards in bridge games. The best players are great at this and almost seem to see through your cards.
The idea is mainly to track how different card suits are distributed and how many high cards the opponent’s got. The easiest way to start counting cards at bridge is to focus on one suit at first and then move on to two, three and, finally, maybe even four suits.
Two details should be remembered when tracking cards in bridge: there are 13 cards of each suit and 13 cards in each hand.
As I’m less than an amateur at bridge, it’s better for you to learn about this subject from experienced players. This page seems to explain bridge card-tracking better than any other guide that I’ve found: Counting Out a Bridge Hand.