Those who want to win the progressive jackpot should play as many lines as they can afford. This will maximize the amount of time you spend on slot machines, and offer you more chances to win the jackpot. Assuming you're just looking to have some fun, it's best to keep the number of paylines under 5.
If your first two cards total 21, you win because you have a Blackjack which pays 3 to 2 on your bet. If the dealer also has a Blackjack, you would Push (tie).
The name of the game is to beat the Bank, in the person of the Dealer, without exceeding 21. Otherwise you lose your bet. If you reach a Blackjack (21) your bet is multiplied by 3. If you beat the Dealer without reaching 21, you win double your bet.