In most variations of these casino games, you can choose to Hit, Split, Double Down or Stand.
If you Hit you simply get another card. The same thing happens if you Double Down, but you also double the wager that you placed. If you have a Pair of cards with the same value, you can Split it into 2 new hands of a single card, and get another card dealt to both of these.
When you are satisfied with your cards, you Stand and end the round. As you develop your skills, your understanding of various probabilities and outcomes will also deepen and you'll be able to apply this knowledge when you're looking at your cards and planning your Blackjack strategy.