Only in the long run over hundreds of hours of playing can you count on winning. The underlying principle behind card counting is that a deck rich is tens and aces is good for the player, a deck rich in small cards is good for the dealer. The reason for this is that blackjacks are more likely in a deck rich in tens and aces, and they benefit the player more than the dealer. Another reason is that the dealer will break more often in a deck rich in tens. To gauge the richness of the deck in good cards the player will keep track of the cards the are already played.
Strategies vary but all assign a point value to each card.
Everything else is 0, or neutral. Then the counter constantly adds and subtracts from the count according to the cards played. A positive count means that a disproportional number of small cards have already been played which means the deck is rich in large cards. This will tell you the relative richness of the deck in good cards.