You and fellow players are dealt two cards each whilst the dealer is dealt one face up. If your first 2 cards add up to 21 (an Ace and a card valued 10), that's Blackjack! If they have any other total, decide whether you wish to 'draw' or 'stay'. You can continue to draw cards until you are happy with your hand.
In blackjack, players attempt to reach a score of 21 without exceeding it before the dealer hits 17. You can win if you don't bust and your total is higher than the dealer cards. Hitting exactly 21 can mean even bigger winnings.