After you've placed your bet, you and the dealer will each get 2 cards. You'll be able to see both of yours, and you can usually see one of the dealer's too. Based only on this information, you'll need to decide what to do next.
The moves that are allowed vary, but you can usually Hit and get another card, Double Down and get another card while doubling your wager, or Split a Pair into 2 hands, and get an extra card dealt to each.
In formulating your Blackjack strategy, you need to think about how likely different outcomes are, based on your hand, the dealer's cards and how many decks are used. If you are participating in live casino games, the probabilities will change because more cards will have been dealt.