A player total of 21 on the first two cards is a "natural" or "blackjack", and the player wins immediately unless the dealer also has one, in which case the hand ties.
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.