The most important thing to know about blackjack is the basic strategy. 
For every set of blackjack rules in a particular casino, there is an optimal blackjack strategy that would tell you what to do if you have particular hand provided that you see the dealer’s upper card. It is strongly recommended that you play by this strategy in any case to maximize your winging chances in long run this strategy is simply the best way to play every possible situation. At the top of the page is the basic strategy for four or more decks when the dealer hits on soft 17, which is the most common game.
To use the basic strategy look up your hand along the left vertical edge and the dealer’s up card along the top. In both cases an A stands for ace. From top to bottom are the hard totals, soft totals, and splittable hands. Rule variations can have an effect on some borderline situations. The most flexible rules are the number of decks, whether the dealer hits or stands on a soft 17, and whether doubling is allowed after splitting.
Some obvious situations have been left out to keep the chart as small as possible. I have more basic strategy charts for other numbers of decks, as well as European blackjack.
But you most play Blackjack with carefully.